box3d: SIMD (SSE2/NEON), double-precision large world, snapshots, benchmarks

- contact solver wide ops + V32 now have real SSE2 and NEON paths selected
  by target arch; scalar fallback behind the disable-simd feature. All three
  paths are bit-identical (cross-arch determinism verified: same ragdoll
  hash on NEON, SSE2 under Rosetta, and scalar).
- double-precision feature (C BOX3D_DOUBLE_PRECISION): f64 world positions
  with the exact C boundary-function semantics; enables the far-from-origin
  test halves (157 tests in DP mode, 151 default).
- world snapshots: recording substrate subset (buffer/writers/geometry
  registry/readers) + world_snapshot.c port; bit-identical continuation
  after restore, corrupt-image rejection.
- examples/benchmark.rs: all 10 C benchmark scenarios; serial Rust runs
  1.05-1.55x slower than C -O2 at one worker (geomean ~1.3x with fat LTO).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>