VDB → ZVOL importer

Claims .vdb (OpenVDB) writes and produces a sibling .zvol — the engine-native dense RGBA8 volumetric format consumed by the volumetrics system.

This is a standard importer asset: init.luau returns a { canImport, import } module that the importer dispatcher (@builtin::assetTypes.importer) loads lazily and runs only when a matching file is written. It does not execute anything at load time.

Pipeline

1. Real parse — if the openvdb plugin in the builtin library (@builtin::plugins.openvdb) is loaded, the VDB B-tree is parsed into a dense RGBA8 voxel buffer and wrapped in a 64-byte ZVOL header.
2. Procedural fallback — without the plugin, a deterministic noise-modulated torus is synthesised (FNV-1a hash of the first 256 source bytes seeds it) so the importer + demo wiring stays functional. The produced_by field in the result distinguishes the two paths.

ZVOL header (64 bytes)

offset	size	field
0	4	magic ZVOL
4	4	u32 version (1)
8	4	u32 width
12	4	u32 height
16	4	u32 depth
20	4	u32 format (1 = rgba8)
24	12	f32[3] aabb_min
36	12	f32[3] aabb_max
48	16	reserved
64	…	voxel data, w*h*d*4 bytes, XY-major Z-slowest