wgpu: area measurement tool (A hotkey, BFS coplanar patch + cyan highlight)

Ports Bonsai's AreaMeasurement onto WgpuViewportWindow as a new
WgpuAreaMeasurement class. Each LMB pick resolves to (instance,
triangle), BFS-expands the coplanar patch (dot(normal, seed_normal)
> 0.9999, ~0.81° tolerance), and toggles it in/out of the running set.
Alt+LMB skips BFS for single-triangle accumulate. Connected-components
sweep over the selected set produces one "X.XXXX m²" label per patch
at its area-weighted centroid in world space; HUD shows the running
total + triangle count.

Dependencies layered in:

- WgpuOverlayRenderer.setHighlightTriangles / encodeHighlightTriangles:
  translucent world-space triangle list (cyan @ 0.45 alpha), depth-
  tested but depth-write off so the corner gizmo + labels still sit
  on top.
- WgpuViewportWindow.pickMeshLocalAt: reuses pickSurfaceAt for the
  world hit, then inverts the instance's composed transform to express
  it in mesh-local space — what the BFS needs. Uses the live map key
  (`mid`) rather than InstanceCpu.model_id, which is whatever the GL
  streamer wrote at sidecar-write time and goes stale across sessions.
- WgpuViewportWindow.readbackMeshTriangles: CPU mesh shadow lookup.
  The shadow itself is populated during the same dequant pass that
  computes mesh-local volume — applyCachedModel for full loads and
  applyStreamedChunk for streaming, so the BFS has data the moment
  the user can pick it.

WgpuModelGpuData gains a MeshTriangles vector indexed by mesh_id;
doubles per-vertex CPU memory (12 B/vert) but skips wgpu mapAsync
plumbing for now. Bounds-check at pick time gracefully no-ops when a
stale sidecar field is out of range.

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

src/ifcviewer-wgpu/WgpuModelGpuData.h

@@ -288,6 +288,24 @@ struct WgpuModelGpuData {
     // until the chunk holding the mesh has been delivered.
     std::vector<double> mesh_local_volumes;
 
+    // CPU shadow of each mesh's mesh-local positions + LOD0 indices.
+    // Populated at applyCachedModel (or per-chunk in streaming) from
+    // the same raw vertex bytes the volume calc dequantises. The Area
+    // measurement tool reads this directly — no GPU readback, matching
+    // the Volume tool's policy.
+    //
+    // Doubles per-vertex memory (12 B/vert GPU + 12 B/vert CPU). The
+    // alternative is a wgpu mapAsync readback per first-touched mesh,
+    // which adds async plumbing and a per-click stall; pay the memory
+    // upfront instead. Trim by sizing each entry down at population
+    // (reserve exact). For huge federations this can be a real
+    // working-set cost — revisit if it shows up in profiles.
+    struct MeshTriangles {
+        std::vector<float>    positions;  // 3 * vertex_count, mesh-local
+        std::vector<uint32_t> indices;    // 3 * triangle_count, LOD0
+    };
+    std::vector<MeshTriangles> mesh_triangles_cache;
+
     // object_id (globally rebased) → instance index in `instances`.
     // Populated alongside the instance vector so the Volume tool can do
     // O(1) instance lookup instead of linear-scanning every model.