Phase 3B: per-instance LOD via meshoptimizer simplifySloppy

Decimate each unique mesh once at sidecar-build time and swap to the
reduced index slice per-instance per-frame when projected sphere radius
drops below IFC_LOD1_PX (default 30).  Same VBO, same SSBO, just a
different firstIndex/count in the indirect command.

Extends MeshInfo (48→56 B) with lod1_ebo_byte_offset + lod1_index_count
and bumps the sidecar to v5.  buildLods() runs inside
onStreamingFinished, appends decimated indices to sd.indices,
applyLodExtension pushes the EBO suffix to the live GPU state, and the
sidecar is written with LOD1 baked in.

simplifySloppy (voxel clustering) is used instead of the default
edge-collapse meshopt_simplify because BIM brep output is per-triangle-
unwelded and non-manifold after welding — simplify returned the input
unchanged for every mesh tested.  Sloppy ignores topology.  Knobs
(IFC_LOD_SLOPPY, IFC_LOD_ERROR, IFC_LOD_RATIO, IFC_LOD_MIN_SAVINGS,
IFC_LOD_LOCK_BORDER, IFC_LOD_DEBUG) are available for A/B tuning.

Result on the 128M-tri 10-model test scene (GTX 1650, 2px contribution
cull): 20.2 → 43.2 fps, 40M → 14M visible triangles, no change in
object count.  LOD build adds 100–600 ms per model on first open,
cached thereafter.

README Phase 3B section is now a full writeup of pipeline, selection,
decimator-choice rationale, env vars, and measured numbers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/SidecarCache.h

@@ -34,7 +34,10 @@
 #include <memory>
 
 static constexpr uint32_t SIDECAR_MAGIC   = 0x49465657;  // "IFVW"
-static constexpr uint32_t SIDECAR_VERSION = 4;
+// v5 = MeshInfo extended with lod1_ebo_byte_offset + lod1_index_count (56 B).
+//      sd.indices may contain an appended LOD1 index slice for each mesh
+//      where meshoptimizer decimation produced useful output.
+static constexpr uint32_t SIDECAR_VERSION = 5;
 static constexpr uint32_t SIDECAR_ENDIAN  = 0x01020304;
 
 // Fixed-size element record.  Strings are stored as (offset, length) pairs