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/AreaMeasurement.cpp

@@ -22,7 +22,7 @@
 #include "OverlayRenderer.h"
 #include "ViewportWindow.h"
 
-#include <QDebug>
+#include <cstdio>
 #include <QString>
 
 #include <algorithm>
@@ -280,9 +280,10 @@ void AreaMeasurement::onPick(ViewportWindow& vp,
 
     rebuildHighlightAndLabels(vp);
 
-    qInfo("[wgpu area] %s%.6f m^2  (total: %.6f m^2, %zu tris)",
-          delta >= 0.0 ? "+" : "", delta,
-          total_area_m2_, selected_.size());
+    std::fprintf(stderr,
+        "[info] [wgpu area] %s%.6f m^2  (total: %.6f m^2, %zu tris)\n",
+        delta >= 0.0 ? "+" : "", delta,
+        total_area_m2_, selected_.size());
 }
 
 void AreaMeasurement::rebuildHighlightAndLabels(ViewportWindow& vp) {