Add SeeFastSign, a provably-exact sign-only shortcut for SEE(), and wire it into GetMoveSee's cache-miss path

GetMoveSee's only consumer, ComputeMoveExtension, reads iMoveSee in
exactly two places -- both plain sign checks (`< 0` / `>= 0`), never
the magnitude. On a cache miss GetMoveSee always paid for a full SEE()
exchange walk (list-build, heap, x-ray discovery) just to throw away
everything but the sign.

SeeFastSign answers the sign question directly, without building
either SEE_LIST: if the defending side has zero attackers on the
target square, SEE()'s own exchange loop terminates on its first
iteration anyway and returns exactly captured+promoted, which is never
negative -- so "no defender" provably implies SEE >= 0, not an
approximation. Falls back to real SEE() whenever a defender exists.

Must check with mv.cFrom removed from occupancy, not the raw pre-move
board -- the mover's own departure can unmask a defender that's
currently blocked by the mover itself sitting on the same ray (exactly
what SEE()'s own _AddXRaysMailbox discovers explicitly). A first
version missed this and returned a wrong TRUE for a quiet bishop move
that unmasked a queen's diagonal attack through the vacated square,
caught by the correctness test below before it ever reached production
code.

testsee.c's TestSeeFastSign asserts SeeFastSign's sign matches real
SEE() across 3000 random positions x every legal move, then benchmarks
both: SeeFastSign runs at 0.40x-0.65x real SEE()'s cycles/call.

Measured end-to-end (sd10 + sn5M, all three curated suites, before vs.
after): node counts are bit-for-bit identical on every suite/depth
pair, as expected from a provably sign-equivalent function -- zero
search-behavior change, no self-play gate needed. Wall-clock improved
on all 6 measurements, ~2.7% overall.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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