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* Speed up TurboQuant Hadamard rotation by ~3x Profiling apply_inverse showed 44% of cycles in the Fisher-Yates swap replay (serial LCG plus a 64-bit modulo per element), not in the WHT. - Materialize the shared permutations as index maps once in HadamardRotation::new; each apply-time permutation becomes a flat gather pass ping-ponging between the vector and a thread-local scratch buffer. The replay path stays as the cfg(test) parity oracle. - Fuse consecutive WHT outer butterfly stage pairs (h, 2h) into one pass over the array (AVX2), halving memory traffic for h >= 16. - Fuse the normalization multiply into the transform's final-stage stores (wht_dispatch_scaled), removing the separate normalize pass. Output is bit-identical on all paths: pinned by the existing struct-vs-replay and SIMD-vs-scalar bit-equal tests plus a new wht_dispatch_scaled parity test. NEON is unchanged. Criterion hadamard bench (Zen 5): apply 2.8-3.7x faster across dims 128-4096 (1024: 6.98us -> 2.03us), apply_inverse 2.8-3.5x (1024: 6.10us -> 2.05us). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> * Address review: harden gather contract, small-dim parity, grow-only scratch - Mark gather_permuted as unsafe fn: the get_unchecked justification relied on a caller invariant (map indices in range) that the signature did not surface; document it as a # Safety contract instead. - Add dims 5 and 50 to static_rotation_matches_struct_and_roundtrips to pin the map path against the replay oracle on degenerate chunk splits. - Make the thread-local gather scratch grow-only, so threads alternating between dims no longer shrink and re-zero the buffer on every call. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> * Hard-assert input length in apply/apply_inverse Fail fast at the public boundary: with debug_assert only, a release build would WHT-normalize a wrong-length slice before the gather's hard length asserts panic. Flagged by CodeRabbit. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> --------- Co-authored-by: Claude Fable 5 <noreply@anthropic.com>