GPU instancing: streamer, viewport, shaders rewritten

Commit A of the instancing migration (Phase 3a).  The streamer now runs
the iterator with use-world-coords=false and dedupes by the geometry's
representation id, emitting a MeshChunk once per unique geometry and an
InstanceChunk per placement.  The viewport keeps geometry in local
coordinates (28 B/vertex, down from 32) and applies the per-instance
transform in the vertex shader via an std430 SSBO indexed by
gl_InstanceID + a per-draw uniform offset.  After streaming finishes
finalizeModel() stable-sorts instances by mesh_id, assigns each mesh a
contiguous range, and uploads the SSBO; render then issues one
glDrawElementsInstancedBaseVertex per mesh.

BvhAccel is reshaped to operate on a generic BvhItem (world AABB +
model_id) so it can drive instance-level culling, but the path is not
wired in yet -- every instance is drawn every frame in this commit.
Progressive-during-streaming rendering is likewise disabled: a model
appears when its SSBO is uploaded, not incrementally.  Sidecar cache
is stubbed (reads miss, writes are no-ops); the v4 on-disk format with
MeshInfo + InstanceGpu sections lands in Commit B.

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

src/ifcviewer/MainWindow.h

@@ -62,7 +62,8 @@ private slots:
     void onFileOpen();
     void onFileSettings();
     void onProgressChanged(int percent);
-    void onElementReady(UploadChunk chunk);
+    void onMeshReady(MeshChunk chunk);
+    void onInstanceReady(InstanceChunk chunk);
     void onStreamingFinished();
     void onObjectPicked(uint32_t object_id);
     void onTreeSelectionChanged();