Rust bitboard engine: parity gates and throughput benchmark vs TypeScript and C++ engines

The Rust bitboard engine is trace-equivalent to the frozen C++ reference, the proven C++ fast engine, and the TypeScript engine on every observable, and is the fastest of the three. Board representation is seven u32 column words at 4 bits per cell (28 bytes): gravity is a single PEXT bit-gather per column, a row rise is (word << 4) | SOLID, and cover hits are counted board-wide with a 4-way bitboard parallel counter. All parity gates pass with zero mismatches: 3 trajectory arms (512 center + 256 search-policy games vs C++ playHeadlessMove; 256 games vs the TypeScript seededRandom driver) totalling 36,427 moves and 40,286 waves; 150,854 leaf states bit-identical as uint64 patterns; 105 d4s7 and 10 d5s7 roots with bit-identical per-column values and identical actions; the values gate re-run with the transposition table enabled proves cache-independence. Measured on the shared AMD Ryzen AI MAX+ 395 workstation (best-of-N, load 1.1-1.7): single-core engine throughput 12.8M moves/s vs C++ fast 6.5M (1.97x) and TypeScript 0.65M (19.8x); leaf 155.6 ns vs C++ fast 187.5 ns (1.20x); fair search at d4s7 908 ms/decision vs C++ fast 1,071 ms (1.18x) with a 3.1 MB direct-mapped table vs the C++ 16.2 MB LRU; d5s7 7,047 ms at 1M entries vs 7,817 ms (1.11x). Game-level scaling is shared-nothing and near-linear (10.3x on 16 physical cores on the shared machine; 14.1x in a clean run), with identical results at every worker count. A key recorded finding: the transposition table's 1.3% node hit rate is misleading — each hit prunes a whole subtree, so the table eliminates ~47% of work at d4s7 and ~90% at d5s7, and a cheap direct-mapped depth-preferred table captures nearly all of the strict-LRU table's payoff at a fifth of the memory. No strength claim; no new seeds opened.