wgpu chunks: pack LOD1 indices alongside LOD0; cull picks per-instance

Sidecar LOD1 is per-mesh, index-only — meshoptimizer-baked decimated
index slices that share the LOD0 VBO. Before this change the wgpu
chunk path force-disabled LOD1 (effective_lod1 = false; tagged in a
comment as "until per-chunk LOD1 storage lands"), so it had to walk
every visible instance at full LOD0 even when the per-instance LOD
selection said the projected radius was below the LOD1 threshold.

Per-chunk layout: append LOD1 indices for the chunk's meshes after
all LOD0 indices in the same pool slice. A new
mesh_chunk_local_lod1_first_u32 array records each mesh's LOD1
starting offset (in u32s) within the chunk's index slice; LOD0
offsets stay where they were. The cull's emit then sets
VisibleDrawGpu.ebo_first_u32 to whichever side matches the per-
instance use_lod1 decision the prior #8 commit already computed.
Vertex pulling is oblivious to the LOD split — it just reads the
indices the cull pointed it at.

c.index_count is repurposed as the LOD0+LOD1 total so the pool
allocation, eviction's pool-fit check, and the VRAM accounting all
scale automatically. c.lod1_index_count exposes the LOD1 portion
for stats.

makeChunkRequest appends LOD1 byte ranges to req.i_ranges in the
same per-mesh order; the streaming worker concatenates ranges in
order, so the assembled idx blob lands LOD0-first / LOD1-second
which matches the chunk-local packing.

On a 10-model regen with meshoptimizer-baked sidecars, the bench
[frame] heartbeat shows lod1 firing on ~80-90% of LOD1-eligible
instances and saving 10-30M tris per frame versus the prior
LOD0-only ceiling. Same camera/scene on basic.ifc is still
pixel-identical (no mesh in basic.ifc is large enough to bake a
LOD1, so the cull just follows the LOD0 path it always did).

Temporary debug counters (lod1_dbg_count_ et al.) print
"lod1 X/Y (saved Z tris, N no-lod1)" in both interactive and
benchmark [frame] heartbeats — kept on while LOD1 correctness gets
confirmed across more scenes, will come out once trust is built.

The non-streaming applyCachedModel path runs the same LOD1 plumbing
but no longer fits the full federation scene in pool (extra index
bytes push past the 2 GB single-buffer cap); that mode was
already streaming-only on that scene before.

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

src/ifcviewer-wgpu/WgpuViewportWindow.h

@@ -530,6 +530,15 @@ private:
     // Tick count for the interactive (non-bench) [frame] heartbeat log.
     // Increments every render() and prints stats every N frames.
     int      interactive_frame_count_ = 0;
+
+    // Per-frame LOD selection counts, mutated from cullModelCpuCompute
+    // and reset after the [frame] heartbeat prints them. Keeps an eye
+    // on whether LOD1 is actually firing on real scenes — early-days
+    // diagnostic while we trust the new code path.
+    mutable uint32_t lod1_dbg_count_              = 0;
+    mutable uint32_t lod0_dbg_eligible_count_     = 0;
+    mutable uint32_t lod0_dbg_no_lod1_count_      = 0;
+    mutable uint64_t lod1_dbg_tris_saved_         = 0;
 };
 
 #endif // WGPUVIEWPORTWINDOW_H