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/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")) {