ResultE-FAST-M6: port M=6 reveal sampling into the fast memo engine, trace-equivalent to the native factored search

E-FAST-M6 passes: factored reveal sampling (N disc strata x M reveal samples, native scenario indexing s = r*N + d over T = N*M) is ported into the fast memo engine as drop7::fastr::FastFactoredSearch and is trace-equivalent to the native FactoredSearch. Grid gate: 6,300 live probe decisions (525 per point over d3/d4 x N5/N7 x M1/M2/M6) with 0 column, 0 work-count and 0 completed-depth mismatches, including 549 work-limited decisions on the two budget-capped d4-M6 points where both engines degrade to completed depth 3 identically. All 3 retained C0 games replay to byte-for-value final identity with the fast engine driving (335 decisions, 0 mismatches). M=1 regression: 2,100 decisions x {memo-on, memo-off} bit-identical to the untouched fast::FastSearch on all six metric fields. Determinism byte-identical across repeated runs and {26,7,1} threads; mirror invariance exact with symmetric boards excluded (finding-13 4C; one disclosed gate-harness iteration); the one-entry leaf memo stays enabled under M>1 (board-memcmp keying cannot alias across reveal samples) with memo-on/off trace identity across the grid. Continuation duty: 8 CRN continuations (2 C0 roots, K=4, H=40) byte-identical to native. Realised speedup at d3 N7M6 is 5.5-5.8x (play duty 2.140 -> 0.385 s/move; continuation duty 0.990 -> 0.177 s/move; grid 4.38-5.87x across points), measured under the gate's own 26-thread load - well below the 10-40x hoped for in the hypothesis, because that figure divided native M6 seconds by fast M1 seconds and ignored the ~27x work ratio. P-SOL continuation labels at native D3 N7M6 semantics now cost ~0.18 s/move instead of ~1-3.4 s/move.