Class TileDecodePool

True once the pool has fatally failed (a worker couldn't start / errored); the caller should fall back to inline decode from here on.

A worker per core, less one for the main thread, capped at 4 (decode is the bottleneck, not parallelism — and each worker holds its own WASM decoders).

Decodes one unwrapped glb's geometry off-thread into packed, transferable primitives. smoothNormals opts normal-less primitives into smooth (vs the spec-default flat). Rejects if the pool is unavailable (caller falls back to inline).