wgpu cull: chunk-level frustum cull replaces BVH walk

cullModelCpuCompute previously had two paths: a flat linear scan over
all instances (default), or a BVH-stack walk (--bvh, gated off because
it regressed on dense scenes — the BVH built per instance but its
interior-node AABBs spanned huge chunks of model so most subtrees
straddled the frustum and the walk overhead beat the rejection win).

With spatial chunk planning (commit 4d3617420) chunks ARE already a
one-level spatial partition of the model, with tight per-chunk AABBs.
So the same wholesale-reject behaviour falls out of just walking
m.chunks: frustum-test each chunk's AABB once, and on hit, iterate
its (new) instance_ids list. No per-node traversal overhead, no
dependency on rebuilding a BVH alongside the chunk plan.

Changes:
- Chunk gains an instance_ids vector, populated in both apply paths
  alongside the per-chunk AABB accumulation.
- cullModelCpuCompute drops the if-bvh / else-linear-scan dichotomy
  in favour of `for chunk: frustum-test then iterate c.instance_ids`.
- Per-model ModelBvh field, buildModelBvhOne call sites, BvhAccel.cpp
  in CMakeLists, bvh_enabled_ field, and --bvh CLI flag all removed —
  dead code now that chunk-cull subsumes them.
- BvhAccel.{h,cpp} stay in src/ifcviewer for the GL backend's use.

Benchmark (big federation, --streaming, close camera): avg 37 fps
(was 36) / median 53 (was 53). Same order on the metric — the
parallelism across models was already amortising frustum-check cost,
so the per-chunk early-out saves only fragments of cull wall time.
Real cull-perf win will come from chunk-level HiZ (potentially) or
GPU compute cull (task #17). What this commit really delivers is
architectural simplification + removal of a dead-but-not-dropped
code path.

Pixel-identical to non-streaming on basic.ifc.

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

src/ifcviewer-wgpu/WgpuViewportWindow.h

@@ -394,14 +394,6 @@ public:
     // scene fits through the constraints a browser will impose.
     bool web_limits_ = false;
 
-    // BVH-walk cull. Default OFF: the BVH adds ~17ms walk overhead on
-    // dense centred-camera scenes without rejecting enough subtrees to
-    // compensate (every subtree's AABB straddles the frustum). It MAY help
-    // on spatially-separated scenes (e.g. distant camera looking at one
-    // model in a sprawling federation). Toggle on via --bvh to measure.
-    // Real default-on requires further tuning — see task #15.
-    bool bvh_enabled_ = false;
-
     // Streaming load (task #16). When enabled, queueLoadSidecar routes
     // through the metadata-only reader: mesh dict + instance dict + georef
     // load immediately; per-chunk vertex bytes are read + uploaded on