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-minimal/main.cpp

@@ -23,6 +23,8 @@
 #include <QWidget>
 #include <QVBoxLayout>
 
+#include "Log.h"
+#include "LogQt.h"
 #include "ViewportWindow.h"
 
 // Stage-1 driver: opens a single window with the wgpu viewport embedded,
@@ -85,11 +87,12 @@ int main(int argc, char* argv[]) {
             if (ok) {
                 viewport->setCamera(v[0], v[1], v[2], v[3], v[4], v[5]);
             } else {
-                qWarning() << "--camera: failed to parse" << parser.value("camera");
+                Log::warn() << "--camera: failed to parse "
+                            << parser.value("camera");
             }
         } else {
-            qWarning() << "--camera: expected 6 comma-separated floats, got"
-                       << parts.size();
+            Log::warn() << "--camera: expected 6 comma-separated floats, got "
+                        << parts.size();
         }
     }