Two-sided lighting, rename misleading draw-count stat

Two bugs conflated as "weird colors":

1. Two-sided lighting.  IFC placements often embed reflection
   matrices (mirrored families).  Transforming a_normal by
   mat3(inst.transform) produces a normal pointing the wrong way
   on those instances, and max(n·L, 0) then clamps the surface to
   pure ambient — reads as dark / washed out.  Use gl_FrontFacing
   to flip n in the fragment shader so both winding orientations
   shade correctly.  The proper fix (ship an inverse-transpose
   normal matrix or a det-sign bit per instance) is still owed;
   that would unlock re-enabling GL_CULL_FACE for a big fragment-
   work win on closed solids.

2. Stats label "inst_draws" was counting indirect sub-draws, not
   actual GL draw calls — misleading since MDI collapses N sub-
   draws into one glMultiDrawElementsIndirect.  Split into
   gl_draw_calls (real GL calls, = drawn-model count) and
   indirect_sub_draws (packed sub-commands).  For a BIM model
   with 47k unique meshes at full view this now correctly reads
   "1 gl_draws (47092 sub)" rather than suggesting 47k driver
   dispatches.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/MainWindow.cpp

@@ -41,13 +41,15 @@ MainWindow::MainWindow(QWidget* parent)
     connect(viewport_, &ViewportWindow::frameStatsUpdated, this, [this](const ViewportWindow::FrameStats& s) {
         if (!stats_label_->isVisible()) return;
         stats_label_->setText(
-            QString("%1 fps | %2 ms | %3/%4 obj | %5/%6 tri")
+            QString("%1 fps | %2 ms | %3/%4 obj | %5/%6 tri | %7 gl_draws (%8 sub)")
                 .arg(s.fps, 0, 'f', 1)
                 .arg(s.frame_time_ms, 0, 'f', 1)
                 .arg(s.visible_objects)
                 .arg(s.total_objects)
                 .arg(s.visible_triangles)
-                .arg(s.total_triangles));
+                .arg(s.total_triangles)
+                .arg(s.gl_draw_calls)
+                .arg(s.indirect_sub_draws));
     });
 
     connect(&AppSettings::instance(), &AppSettings::showStatsChanged, this, [this](bool show) {