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-minimal/main.cpp

@@ -54,10 +54,6 @@ int main(int argc, char* argv[]) {
         "Request the WebGPU mandatory floor limits (128MB max storage binding) "
         "instead of the adapter's actual max. Use to verify scenes fit through "
         "browser constraints."});
-    parser.addOption({"bvh",
-        "Enable BVH-walk cull. Off by default — currently a regression on "
-        "dense camera-looking-at-everything scenes; may help on sprawling "
-        "federations where most of the scene is off-screen."});
     parser.addOption({"streaming",
         "Enable streaming sidecar load. Reads metadata-only at load time; "
         "vertex chunks are deferred and loaded on demand as they become "
@@ -68,7 +64,6 @@ int main(int argc, char* argv[]) {
     viewport->resize(1280, 800);
     if (parser.isSet("no-hiz"))     viewport->hiz_enabled_      = false;
     if (parser.isSet("web-limits")) viewport->web_limits_       = true;
-    if (parser.isSet("bvh"))        viewport->bvh_enabled_      = true;
     if (parser.isSet("streaming"))  viewport->streaming_enabled_ = true;
 
     QWidget* container = QWidget::createWindowContainer(viewport);