ifcviewer: quantize VBO to 16 B/vertex (sidecar v6)

Position now u16x3 normalized against each mesh's local AABB; normal
oct-encoded to i16x2; RGBA8 colour unchanged. Per-mesh dequant basis
lives in a new MeshGpu SSBO at binding 2; both main and pick shaders
mix() against it before applying the instance transform.

Drops VBO and sidecar size by ~43 % (28 -> 16 B/vert), which matters
mostly for warm-load downloads of precomputed sidecars and steady-state
VRAM. LodBuilder dequantizes positions into a scratch buffer before
calling meshopt, since meshoptimizer needs float positions.

Also fixes a streaming-time crash in cullAndUploadVisible: bvh_items
was only populated at finalize, but the linear fallback indexes it
during streaming. Mirror BvhItem appends in uploadInstanceChunk so the
hot path stays valid before the BVH is built.

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

src/ifcviewer/README.md

@@ -46,9 +46,12 @@ engine with a Qt6 interface and OpenGL 4.5 rendering.
 - **Per-model GPU buffers**: each loaded model gets its own
   VAO/VBO/EBO/instance-SSBO/visible-SSBO/indirect-buffer. No cross-model
   growth copies. Removing a model frees its GPU memory immediately.
-- **Local-coordinate vertex format (28 B):** position (3 floats) + normal
-  (3 floats) + packed RGBA8 colour (1 uint). The per-instance transform is
-  applied in the vertex shader via an SSBO lookup. No world-baked vertex data.
+- **Quantized local-coordinate vertex format (16 B):** position as
+  `u16x3` normalised against each mesh's local AABB, octahedral-encoded
+  normal as `i16x2`, packed RGBA8 colour. Dequantisation basis is per
+  mesh, uploaded once in a `MeshGpu` SSBO at binding 2. The per-instance
+  transform is applied in the vertex shader. No world-baked vertex data.
+  ~43 % smaller VBO and sidecar than the previous 28 B float layout.
 - **Multi-draw indirect:** every frame the CPU builds a flat list of visible
   instance indices and one `DrawElementsIndirectCommand` per non-empty mesh,
   then issues a single `glMultiDrawElementsIndirect` per model. 50k visible
@@ -93,7 +96,7 @@ engine with a Qt6 interface and OpenGL 4.5 rendering.
 | `InstancedGeometry.h` | Shared structs: `MeshInfo`, `InstanceCpu`, `InstanceGpu`, chunk records |
 | `BvhAccel.h/cpp` | Median-split BVH builder; operates on instance world-AABBs |
 | `LodBuilder.h/cpp` | Post-stream decimation of unique meshes via meshoptimizer (`simplifySloppy`) |
-| `SidecarCache.h/cpp` | Raw binary `.ifcview` (v5) sidecar read/write |
+| `SidecarCache.h/cpp` | Raw binary `.ifcview` (v6) sidecar read/write |
 | `AppSettings.h/cpp` | Persisted preferences (geometry library, stats overlay, backface culling) |
 | `SettingsWindow.h/cpp` | Settings dialog |
 | `CMakeLists.txt` | Build configuration |
@@ -270,7 +273,7 @@ while stack not empty:
 Depth 64 is enough for billions of items on any balanced tree. The stack
 is on the C++ stack, zero per-frame allocation.
 
-#### Sidecar format (`.ifcview`, v5)
+#### Sidecar format (`.ifcview`, v6)
 
 Raw memory dump, Blender-`.blend`-style — no serialisation, no parsing.
 Stores everything needed to skip the `IfcGeom::Iterator` pass:
@@ -278,7 +281,7 @@ Stores everything needed to skip the `IfcGeom::Iterator` pass:
 ```
 SidecarHeader            (magic "IFVW", version, endian, ...)
 uint64_t                 source_file_size
-uint32_t + float[]       vertex data    (7 floats × N_verts, local coords)
+uint32_t + uint8_t[]     vertex data    (16 B/vert quantized; per-mesh basis in MeshInfo)
 uint32_t + uint32_t[]    index data     (mesh-local)
 uint32_t + MeshInfo[]    per-unique-mesh metadata (56 B each, incl. LOD1 slice)
 uint32_t + InstanceCpu[] per-placement records (transform + AABB + ids)
@@ -298,7 +301,8 @@ Per-model state on the GPU:
 
 | Buffer | Contents | Lifetime |
 |--------|----------|----------|
-| `VBO` | Interleaved local-coord vertex data (28 B/vert). One range per unique representation. | Grow-on-demand during streaming; static after finalize. |
+| `VBO` | Quantized local-coord vertex data (16 B/vert: u16x3 pos, oct i16x2 normal, RGBA8). One range per unique representation. | Grow-on-demand during streaming; static after finalize. |
+| `MeshGpu SSBO` (binding 2) | Per-mesh dequant basis (`vec4 aabb_min`, `vec4 aabb_max`). | Grow-on-demand; static after finalize. |
 | `EBO` | Mesh-local uint32 indices. One range per unique representation. | Same. |
 | `SSBO` (binding 0) | `InstanceGpu[]` (80 B each: mat4 transform, object_id, color_override, pad). | Appended during streaming, static after finalize. |
 | `visible SSBO` (binding 1) | `uint32[]` — flat list of visible instance indices, ordered by mesh, uploaded each frame. | Rewritten every frame. |
@@ -311,7 +315,7 @@ struct DrawElementsIndirectCommand {
     uint32_t count;         // mesh.index_count
     uint32_t instanceCount; // visible-list length for this mesh
     uint32_t firstIndex;    // mesh.ebo_byte_offset / 4
-    uint32_t baseVertex;    // mesh.vbo_byte_offset / 28
+    uint32_t baseVertex;    // mesh.vbo_byte_offset / 16
     uint32_t baseInstance;  // offset into the flat visible-index array
 };
 ```