perf(rbd): dedupe shared TriMesh uploads in from_rapier

[haixuanTao]

What

RbdState::from_rapier now caches the GPU Shape descriptor of each TriMesh by its parry shape data pointer. When the same SharedShape Arc is cloned across environments — the normal way to build batched scenes with shared terrain — the flat BVH + pseudo-normals are serialized into shape_buffers once and every clone reuses the descriptor (which only holds ranges into the shared buffers).

Why

Batched RL training scenes routinely attach one large terrain trimesh to hundreds or thousands of environments. Before this, from_rapier re-uploaded the mesh per env: with 3 terrain strips of a few thousand triangles across 4096 envs we measured hundreds of MB of duplicated vertex/index/BVH data; after, it's O(unique meshes) — three uploads total.

Scope / safety

• Cache hit requires the same parry data pointer, i.e. an actual SharedShape clone. Structurally-equal-but-distinct meshes are (correctly) not deduped.
• TriMesh only — every other shape takes the exact previous path, and scenes without shared trimeshes produce byte-identical buffers.
• Host-only; no shader change.

cargo check clean on nexus_rbd3d and nexus_rbd2d. Running in production on our terrain-curriculum training (4096 envs × 3 shared strips) for two days.

🤖 Generated with Claude Code

https://claude.ai/code/session_01U2n9RqmxTJb8UG5d1Sjw4W