Add Chromium-only Blender WebEngine parity work

docs/web/mesh-buffer-layout.md

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+# MeshIR Buffer Layout
+
+This document is the versioned boundary between the Blender reader and the
+Three.js renderer. The compatibility C ABI returns a JSON snapshot. The
+preferred ABI separates JSON metadata from a native WBG1 geometry stream, and
+the Worker transfers each typed mesh buffer without reconstructing large JSON
+arrays. JSON arrays remain a fallback for older engine binaries.
+
+## Coordinate and topology contract
+
+- Blender source coordinates are right-handed, Z-up, with forward `-Y`.
+- Positions and normals use packed little-endian IEEE-754 `float32` values.
+- `indices` are packed little-endian unsigned 32-bit integers and reference
+  `positions` vertices.
+- `topology` is `triangles`; every three indices form one triangle.
+- `triangleCornerIndices` maps every index entry to the original Blender mesh
+  corner. It is required when UV or color attributes are present because a
+  single position can have multiple corner values.
+- `triangleMaterialIndices` has one non-negative integer per triangle and
+  indexes `materialSlotIds`.
+
+## SceneIR fields
+
+| Field | Element count | Type | Domain |
+| --- | ---: | --- | --- |
+| `positions` | `vertexCount * 3` | `float32` | Point |
+| `normals` | `vertexCount * 3` | `float32` | Point, normalized |
+| `indices` | `triangleCount * 3` | `uint32` | Triangle corners |
+| `triangleCornerIndices` | `triangleCount * 3` | `uint32` | Blender Corner |
+| `uvs` | `cornerCount * 2` | `float32` | Corner, optional |
+| `colors` | `cornerCount * 4` | `float32` RGBA | Corner, optional |
+| `triangleMaterialIndices` | `triangleCount` | `uint32` | Face expanded to triangles |
+| `triangleFaceIndices` | `triangleCount` | `uint32` | Source face for edit selection |
+| `edgeVertexIndices` | `edgeCount * 2` | `uint32` | Edge endpoints |
+| `tangents` | `cornerCount * 4` | `float32` | Corner tangent/sign, optional |
+| `splitNormals` | `cornerCount * 3` | `float32` | Corner normal, optional |
+
+`bounds.min` and `bounds.max` are three finite JSON numbers in source object
+space. `materialSlotIds` are stable SceneIR IDs and never contain embedded
+material data.
+
+## Native WBG1 stream
+
+The stream starts with `WBG1`, a little-endian schema version and mesh count.
+Each mesh contains an ID length, eleven element counts, UTF-8 ID bytes, then
+the following arrays in order:
+
+```text
+positions:f32, indices:u32, normals:f32, triangleCornerIndices:u32,
+uvs:f32, colors:f32, triangleMaterialIndices:u32,
+triangleFaceIndices:u32, edgeVertexIndices:u32, tangents:f32,
+splitNormals:f32
+```
+
+Every count is element-based and every scalar is 4 bytes, so array starts stay
+4-byte aligned. The decoder rejects truncation/trailing bytes, unknown mesh
+IDs, count/summary mismatches and indices outside `vertexCount`. Topology
+changes must regenerate triangle-face and edge mappings; stale source arrays
+are never accepted.
+
+`geometryStatus: "binary"` means metadata references a transferred per-mesh
+buffer through `geometryBufferId`. `geometryStatus: "available"` is the JSON
+fallback. After the initial WBG1 generation, `MeshGeometryDelta` carries
+`meshId`, field, byte offset and transferable replacement bytes for stable-size
+changes; topology or attribute-layout changes replace only that mesh envelope.
+Network-style progressive sub-mesh streaming is not implied by WBG1.