ifcviewer: replace qInfo/qWarning with a Qt-free logger seam

Add Log.h (in IfcViewerCore) — a tiny stream-style logger that backs
fprintf(stderr,...), with overloads for the common primitives + char
strings. Mimics qInfo()/qWarning()'s syntax surface enough that
mass-replacing qInfo()→Log::info() and qWarning()→Log::warn() keeps
existing call sites parsing unchanged; .noquote() / .nospace() exist
as compat no-ops so chained qInfo().noquote()<<x<<y patterns survive.

QString streaming is a transitional concern — the QString → std::string
sweep (#80) hasn't landed yet, so ViewportWindow and friends still
construct QStrings for log payloads. LogQt.h (in IfcViewer, not Core)
adds the QString / QStringView operator<< overloads so those streaming
sites work without source changes during the in-flight Qt removal.
When #80 retires QString, LogQt.h drops out.

ViewportWindow.cpp: 132 qInfo/qWarning callsites converted. The two
printf-style qInfo("fmt %s", ...) callsites get fprintf with explicit
[info]/[warn] prefixes to keep the output discoverable.

Also de-Qt'd:
  AreaMeasurement.cpp  — 1 qInfo("fmt", …) → fprintf
  SceneLoader.cpp      — 4 qDebug + 1 qWarning printf-style → fprintf
  GeometryStreamer.cpp — 2 qDebug printf-style → fprintf
  ifcviewer-minimal/main.cpp — 2 qWarning << → Log::warn

Drops <QDebug> from each. Closes #82.

Builds: desktop / bonsai / web all green. Tests 100/100 pass.

src/ifcviewer/GeometryStreamer.cpp

@@ -34,7 +34,7 @@
 #include <limits>
 #include <set>
 
-#include <QDebug>
+#include <cstdio>
 #include <QElapsedTimer>
 
 struct MaterialInfo {
@@ -453,9 +453,10 @@ void GeometryStreamer::run(const std::string& path, int num_threads) {
         return;
     }
     if (!gross_ids.empty()) {
-        qDebug("Excessive voids: %zu element(s) will be loaded without "
-               "opening subtractions",
-               gross_ids.size());
+        std::fprintf(stderr,
+            "[info] Excessive voids: %zu element(s) will be loaded without "
+            "opening subtractions\n",
+            gross_ids.size());
     }
 
     // Shared dedup + AABB state across passes — same geom.id() across
@@ -696,8 +697,9 @@ void GeometryStreamer::run(const std::string& path, int num_threads) {
 
     double dedup_ratio = total_meshes > 0
         ? static_cast<double>(total_shapes) / static_cast<double>(total_meshes) : 1.0;
-    qDebug("Streamer done: %s  %.2fs  shapes=%u  unique_meshes=%u  dedup=%.2fx",
-           path.c_str(), stream_timer.elapsed() / 1000.0,
-           total_shapes, total_meshes, dedup_ratio);
+    std::fprintf(stderr,
+        "[info] Streamer done: %s  %.2fs  shapes=%u  unique_meshes=%u  dedup=%.2fx\n",
+        path.c_str(), stream_timer.elapsed() / 1000.0,
+        total_shapes, total_meshes, dedup_ratio);
     succeeded_ = !cancel_requested_.load();
 }