ifcviewer: de-Qt ViewportWindow public API (QString → std::string)

Take QString out of ViewportWindow's outward-facing surface so it can
eventually move into a Qt-free ViewportCore:

  void     queueLoadSidecar(const QString&)    →  (const std::string&)
  uint32_t loadSidecar(const QString&)         →  (const std::string&)
  QString  cameraString() const                →  std::string …
  void     captureNextFrameToPng(const QString&, bool)
                                               →  (const std::string&, bool)
  void     setHudText(const QString&)          →  (const std::string&)

Internal members also moved off QString:
  std::deque<QString> pending_sidecars_     →  std::deque<std::string>
  QString             pending_screenshot_path_ →  std::string

Implementation strategy: convert at the boundary where ViewportWindow
still leans on Qt internals — `loadSidecar` bridges to QString once
for QFile/QDir/QFileInfo path handling; the screenshot save path
constructs a QString locally for QImage::save; the OverlayRenderer's
HUD setter still takes QString so setHudText converts before calling
through. Each of those bridges goes away when ViewportCore lands and
OverlayRenderer / SceneLoader / SidecarBuilder get their own de-Qt
sweeps. cameraString now produces its CSV via snprintf — no QString
ever instantiated.

Bonsai-side updates (compile-only):
  ifcviewer-minimal/main.cpp — queueLoadSidecar / captureNextFrameToPng
                              callers add .toStdString() on the QString
                              parser result
  modules/viewport/View.cpp  — setHudText callers add .toStdString() to
                              their `QString::arg(...)` formatter chains;
                              two `QString()` empty sentinels become
                              `std::string()`
  Measurement.cpp            — same pattern, two setHudText sites
  ifcviewer/LengthMeasurement.cpp — same, three sites

The cameraString string-streaming fix-up in ViewportWindow.cpp drops
the temporary .toUtf8().constData() bridge from #82 — Log::Stream's
std::string overload now handles it directly.

Builds: desktop / bonsai / web all green. Tests 100/100. Closes #80.

src/bonsaiviewer/Measurement.cpp

@@ -662,7 +662,7 @@ void LengthMeasurement::clear(ViewportWindow& vp) {
     vp.setOverlayPoints({}, 0,0,0,0, 0, 0,0,0,0, 0);
     vp.setOverlayLines({});
     vp.setOverlayLabels({});
-    vp.setHudText(QString());
+    vp.setHudText(std::string());
 }
 
 void LengthMeasurement::onPick(ViewportWindow& vp, int x, int y, bool /*alt*/) {
@@ -805,7 +805,7 @@ void LengthMeasurement::rebuildOverlay(ViewportWindow& vp) {
 
     vp.setOverlayLines(groups);
     vp.setOverlayLabels(labels);
-    vp.setHudText(formatReadout());
+    vp.setHudText(formatReadout().toStdString());
 }
 
 namespace {
@@ -1029,7 +1029,7 @@ void LengthMeasurement::rebuildLaserOverlay(ViewportWindow& vp) {
     pushDots(vp, std::vector<float>(wp, wp + 3));
     vp.setOverlayLines(groups);
     vp.setOverlayLabels(labels);
-    vp.setHudText(hud_lines.join('\n'));
+    vp.setHudText(hud_lines.join('\n').toStdString());
 }
 
 QString LengthMeasurement::formatReadout() const {

src/bonsaiviewer/modules/viewport/View.cpp

@@ -80,7 +80,7 @@ ViewportView::ViewportView(bonsaiviewer::SessionState* session_state,
             area_measurement_->onPick(*viewport_, x, y, alt);
             viewport_->setHudText(QString("Area: %1 m²  (%2 tris)")
                 .arg(area_measurement_->totalArea(), 0, 'f', 4)
-                .arg(area_measurement_->triangleCount()));
+                .arg(area_measurement_->triangleCount()).toStdString());
             break;
         case ViewportWindow::ToolMode::Length:
             length_measurement_->onPick(*viewport_, x, y, alt);
@@ -96,7 +96,7 @@ ViewportView::ViewportView(bonsaiviewer::SessionState* session_state,
         length_measurement_->clear(*viewport_);
         switch (mode) {
         case ViewportWindow::ToolMode::NoTool:
-            viewport_->setHudText(QString());
+            viewport_->setHudText(std::string());
             viewport_->setOverlayLabels({});
             session_state_->setStatusMessage("Measure", "Measurement tool off");
             break;
@@ -241,7 +241,7 @@ void ViewportView::updateVolumeReadout() {
 
     const auto& sel = viewport_->selection().selectionIds();
     if (sel.empty()) {
-        viewport_->setHudText(QString());
+        viewport_->setHudText(std::string());
         viewport_->setOverlayLabels({});
         return;
     }
@@ -268,7 +268,7 @@ void ViewportView::updateVolumeReadout() {
     viewport_->setHudText(QString("Volume: %1 m³  (%2 object%3)")
         .arg(total, 0, 'f', 4)
         .arg(per_obj.size())
-        .arg(per_obj.size() == 1 ? "" : "s"));
+        .arg(per_obj.size() == 1 ? "" : "s").toStdString());
     viewport_->setOverlayLabels(labels);
 }
 

src/ifcviewer-minimal/main.cpp

@@ -75,7 +75,7 @@ int main(int argc, char* argv[]) {
     // Queue sidecars; they're loaded after wgpu init completes in
     // exposeEvent. Ordering matches the command line.
     for (const QString& path : parser.positionalArguments()) {
-        viewport->queueLoadSidecar(path);
+        viewport->queueLoadSidecar(path.toStdString());
     }
 
     if (parser.isSet("camera")) {
@@ -97,7 +97,7 @@ int main(int argc, char* argv[]) {
     }
 
     if (parser.isSet("screenshot")) {
-        viewport->captureNextFrameToPng(parser.value("screenshot"),
+        viewport->captureNextFrameToPng(parser.value("screenshot").toStdString(),
                                         /*quit_after=*/true);
     }
     if (parser.isSet("benchmark")) {

src/ifcviewer/LengthMeasurement.cpp

@@ -356,7 +356,7 @@ void LengthMeasurement::clear(ViewportWindow& vp) {
     vp.setOverlayPoints({}, 0,0,0,0, 0, 0,0,0,0, 0);
     vp.setOverlayLines({});
     vp.setOverlayLabels({});
-    vp.setHudText(QString());
+    vp.setHudText(std::string());
 }
 
 void LengthMeasurement::onPick(ViewportWindow& vp,
@@ -486,7 +486,7 @@ void LengthMeasurement::rebuildOverlay(ViewportWindow& vp) {
 
     vp.setOverlayLines(groups);
     vp.setOverlayLabels(labels);
-    vp.setHudText(formatReadout());
+    vp.setHudText(formatReadout().toStdString());
 }
 
 void LengthMeasurement::rebuildLaserOverlay(ViewportWindow& vp) {
@@ -679,7 +679,7 @@ void LengthMeasurement::rebuildLaserOverlay(ViewportWindow& vp) {
     pushDots(vp, std::vector<float>(wp, wp + 3));
     vp.setOverlayLines(groups);
     vp.setOverlayLabels(labels);
-    vp.setHudText(hud_lines.join('\n'));
+    vp.setHudText(hud_lines.join('\n').toStdString());
 }
 
 QString LengthMeasurement::formatReadout() const {

src/ifcviewer/ViewportWindow.cpp

@@ -689,7 +689,7 @@ void ViewportWindow::setBackgroundColor(const QColor& color) {
 // Sidecar load + GPU upload
 // -----------------------------------------------------------------------------
 
-void ViewportWindow::queueLoadSidecar(const QString& path) {
+void ViewportWindow::queueLoadSidecar(const std::string& path) {
     if (wgpu_initialized_) {
         loadSidecar(path);
     } else {
@@ -697,7 +697,13 @@ void ViewportWindow::queueLoadSidecar(const QString& path) {
     }
 }
 
-uint32_t ViewportWindow::loadSidecar(const QString& path) {
+uint32_t ViewportWindow::loadSidecar(const std::string& path_std) {
+    // Internal implementation still uses Qt's path helpers (QDir tilde
+    // expansion, QFile readability checks, QFileInfo for absolute resolve).
+    // Bridging at the entry boundary keeps the public API Qt-free without
+    // a full internal rewrite — those will move to std::filesystem when
+    // ViewportCore lands (#84).
+    const QString path = QString::fromStdString(path_std);
     if (!wgpu_initialized_) {
         Log::warn().noquote() << "loadSidecar called before wgpu init:" << path;
         return 0;
@@ -1502,7 +1508,7 @@ void ViewportWindow::frameOnFederatedOrigin(uint32_t model_id,
 
 void ViewportWindow::flushPendingSidecarQueue() {
     while (!pending_sidecars_.empty()) {
-        const QString p = pending_sidecars_.front();
+        const std::string p = pending_sidecars_.front();
         pending_sidecars_.pop_front();
         loadSidecar(p);
     }
@@ -3109,8 +3115,11 @@ void ViewportWindow::setOverlayLabels(
     if (isExposed()) requestUpdate();
 }
 
-void ViewportWindow::setHudText(const QString& text) {
-    overlays_.setHudText(text);
+void ViewportWindow::setHudText(const std::string& text) {
+    // OverlayRenderer still uses QString internally (Qt's QImage/QPainter
+    // rasterizes the HUD text). Conversion at the boundary keeps the
+    // public API Qt-free; OverlayRenderer's de-Qt comes later.
+    overlays_.setHudText(QString::fromStdString(text));
     if (isExposed()) requestUpdate();
 }
 
@@ -5011,7 +5020,7 @@ void ViewportWindow::render() {
     // ---- Optional capture: encode copy on the same command buffer -------
     WGPUBuffer    capture_buffer    = nullptr;
     uint32_t      capture_padded_bpr = 0;
-    const bool    want_capture       = !pending_screenshot_path_.isEmpty();
+    const bool    want_capture       = !pending_screenshot_path_.empty();
     if (want_capture) {
         const uint32_t row_bytes_unpadded = uint32_t(configured_w_) * 4u;
         capture_padded_bpr = uint32_t(
@@ -5100,11 +5109,12 @@ void ViewportWindow::render() {
             }
             wgpuBufferUnmap(capture_buffer);
 
-            if (img.save(pending_screenshot_path_, "PNG")) {
-                Log::info().noquote() << "[wgpu] saved screenshot:"
-                                  << pending_screenshot_path_ << "(" << w << "x" << h << ")";
+            const QString qpath = QString::fromStdString(pending_screenshot_path_);
+            if (img.save(qpath, "PNG")) {
+                Log::info().noquote() << "[wgpu] saved screenshot: "
+                                  << pending_screenshot_path_ << " (" << w << "x" << h << ")";
             } else {
-                Log::warn().noquote() << "[wgpu] QImage::save failed for"
+                Log::warn().noquote() << "[wgpu] QImage::save failed for "
                                      << pending_screenshot_path_;
             }
         }
@@ -6524,7 +6534,7 @@ void ViewportWindow::driveStreamingLoads() {
         // wait would let the window manager re-layout the window while we
         // wait, capturing at the wrong size. With sync loads the chunk
         // appears in the same frame we enqueue, no deferred-state to manage.
-        if (!pending_screenshot_path_.isEmpty()) {
+        if (!pending_screenshot_path_.empty()) {
             if (loadChunkBytesAndUploadGpu(*cand.m, cand.ci)) {
                 ++enqueued;
                 c.last_visible_frame_idx = streaming_frame_idx_;
@@ -7002,14 +7012,12 @@ void ViewportWindow::toggleProjection() {
     if (isExposed()) requestUpdate();
 }
 
-QString ViewportWindow::cameraString() const {
-    return QString("%1,%2,%3,%4,%5,%6")
-        .arg(camera_target_[0], 0, 'f', 4)
-        .arg(camera_target_[1], 0, 'f', 4)
-        .arg(camera_target_[2], 0, 'f', 4)
-        .arg(camera_distance_,  0, 'f', 4)
-        .arg(camera_yaw_deg_,   0, 'f', 2)
-        .arg(camera_pitch_deg_, 0, 'f', 2);
+std::string ViewportWindow::cameraString() const {
+    char buf[128];
+    std::snprintf(buf, sizeof(buf), "%.4f,%.4f,%.4f,%.4f,%.2f,%.2f",
+                  camera_target_[0], camera_target_[1], camera_target_[2],
+                  camera_distance_, camera_yaw_deg_, camera_pitch_deg_);
+    return std::string(buf);
 }
 
 void ViewportWindow::enterFpsMode() {
@@ -7202,7 +7210,7 @@ void ViewportWindow::applyNavPreset(const char* name) {
 #include <QImage>
 #include <QCoreApplication>
 
-void ViewportWindow::captureNextFrameToPng(const QString& path, bool quit_after) {
+void ViewportWindow::captureNextFrameToPng(const std::string& path, bool quit_after) {
     pending_screenshot_path_ = path;
     pending_screenshot_quit_ = quit_after;
     if (isExposed()) requestUpdate();
@@ -7773,7 +7781,7 @@ void ViewportWindow::keyPressEvent(QKeyEvent* event) {
         return;
     }
     if (key == Qt::Key_C && !(mods & Qt::ControlModifier)) {
-        Log::info() << "--camera " << cameraString().toUtf8().constData();
+        Log::info() << "--camera " << cameraString();
         return;
     }
     // Standard axis-aligned views: X/Y/Z look from +axis, Shift+X/Y/Z from

src/ifcviewer/ViewportWindow.h

@@ -25,7 +25,7 @@
 #include <QElapsedTimer>
 #include <QPoint>
 #include <QSet>
-#include <QString>
+#include <string>
 #include <QTimer>
 
 #include <webgpu/webgpu.h>
@@ -95,14 +95,14 @@ public:
     // call before the window is exposed. The path is resolved against the
     // working directory and read via SidecarCache::readSidecar (which
     // normalises stem → .ifcview).
-    void queueLoadSidecar(const QString& path);
+    void queueLoadSidecar(const std::string& path);
 
     // Synchronous metadata load + GPU upload. Requires wgpu init to have
     // completed (i.e. the window has been exposed at least once). Returns
     // the assigned model_id, or 0 on failure. Reads metadata only (mesh
     // dict + instance dict + georef); per-chunk vertex / index bytes are
     // read on demand by the per-frame loader as chunks become visible.
-    uint32_t loadSidecar(const QString& path);
+    uint32_t loadSidecar(const std::string& path);
 
     // Allocates per-chunk small buffers and the model-shared mesh /
     // instance storage upfront, but leaves each chunk's pool ranges
@@ -172,7 +172,7 @@ public:
     void    focusOnSelectedObject();
     void    toggleProjection();
     bool    projectionOrtho() const { return projection_ortho_; }
-    QString cameraString() const;
+    std::string cameraString() const;
 
     // Snapshot of the orbit camera. Mirrors GL ViewportWindow::CameraState
     // so bonsai's "save view" / "restore view" commands port unchanged.
@@ -250,7 +250,7 @@ public:
     // `quit_after` is true, QCoreApplication::quit() is called once the
     // PNG is written. Use this for headless verification and pixel-diff
     // parity testing against the GL backend.
-    void captureNextFrameToPng(const QString& path, bool quit_after = true);
+    void captureNextFrameToPng(const std::string& path, bool quit_after = true);
 
     // Benchmark mode: render N timed frames (after a small warmup), yaw-
     // sweeping the camera at 0.5°/frame, then print a stats block on
@@ -390,7 +390,7 @@ public:
                            float stroke_b, float stroke_a,
                            float stroke_extra);
     void  setOverlayLabels(const std::vector<OverlayRenderer::Label>& labels);
-    void  setHudText(const QString& text);
+    void  setHudText(const std::string& text);
     // Translucent world-space triangle overlay (Area-tool patch shading).
     // Empty list disables; color is RGBA in [0, 1].
     void  setHighlightTriangles(const std::vector<float>& world_xyz,
@@ -1023,7 +1023,7 @@ private:
     uint32_t next_object_id_ = 1;
 
     // Sidecar paths queued before init completes.
-    std::deque<QString> pending_sidecars_;
+    std::deque<std::string> pending_sidecars_;
 
     // Direct-IFC staging buffers, keyed by streamer model_id. Populated
     // by uploadMeshChunk / uploadInstanceChunk; consumed and cleared by
@@ -1057,7 +1057,7 @@ private:
     bool  last_cull_was_motion_    = false;
 
     // Pending one-shot screenshot, captured at the end of the next render().
-    QString pending_screenshot_path_;
+    std::string pending_screenshot_path_;
     bool    pending_screenshot_quit_ = false;
 
     // Mouse navigation state. LMB drag orbits, MMB drag pans, wheel zooms.