wgpu backend: vertex-pulling main render pass

Stage 3 of the wgpu port. Replaces the clear-only render loop with the
full main shading pass:

  - WGSL port of the GL main shader. Vertex-pulling: the vertex storage
    buffer is read as array<u32> in the shader, with pos/normal/color
    decoded manually per vertex. baseVertex (set per draw to mesh's
    vertex offset) folds into @builtin(vertex_index) automatically;
    firstInstance carries the instance slot for @builtin(instance_index).
    No vertex-input layout — vertex pulling means no IA bindings.

  - Render pipeline bound to depth-32-float (write-on, less compare),
    back-face cull, CCW front face. Pre-multiplies a [-1,1]→[0,1] z-remap
    matrix onto Qt's projection so WebGPU's clip-z convention is met.

  - Two bind groups: group=0 per-frame (uniform with view-proj + key/fill
    light + hemisphere ambient), group=1 per-model (three read-only
    storage buffers: vertices, mesh quant, instances).

  - Depth texture is created lazily and recreated on surface resize.

  - Orbit camera state on WgpuViewportWindow with viewAll() that frames
    the union of all loaded models' world AABBs after the first load.
    Mouse navigation lands later.

  - Draw loop: one drawIndexed per (mesh, instance) pair per model. This
    is correct but CPU-heavy on dense scenes; stage 6 introduces the cull
    + compacted visible list that lets multiple instances of one mesh
    collapse to a single call, and the eventual GPU-driven cull (post
    sunset of the GL backend) goes further.

Verified on /tmp/quad_v13.ifcview (1 mesh, 1 instance) and on a real v13
sidecar baked from basic.ifc via the GL minimal viewer (3 meshes,
3 instances, 864 B verts). No wgpu validation errors fire across pipeline
creation, depth attachment, bind groups, or the draw loop on either.
Visual confirmation deferred until --screenshot lands (task #10) which
is being pulled forward next so we don't keep flying blind.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-wgpu/WgpuModelGpuData.h

@@ -51,6 +51,10 @@ struct WgpuModelGpuData {
     // recompose from placement_transformation when stage matrices change.
     WGPUBuffer instance_storage = nullptr;
 
+    // Bind group binding the three storage buffers above (group=1 in the
+    // main pipeline). Built in applyCachedModel after the buffers exist.
+    WGPUBindGroup bind_group = nullptr;
+
     // Size mirrors for stats / range checks.
     size_t   vertex_bytes   = 0;
     uint32_t index_count    = 0;