NNUE accumulator struct scaffolding; fix 3 bugs found by DEBUG+TEST; add that gate

Partial checkpoint on the nnue-accumulator-plumbing branch, before the
MakeMove/UnmakeMove hookup itself:

- chess.h: NNUE_ACCUMULATOR struct (256 floats, pre-activation, float32
  for now -- quantization to int16 deliberately deferred until a real
  quantization-aware export pass exists, per docs/NNUE.md). Added the
  live, always-current SEARCHER_THREAD_CONTEXT.sNnueAccum[2] (one per
  color) and PLY_INFO.sNnueAccumBackup (the king-move full-recompute
  backup slot, DEBUG-only color tag declared for a later ASSERT).
  LIGHTWEIGHT_SEARCHER_CONTEXT also mirrors sNnueAccum[2] at the same
  relative position -- omitting that broke its byte-compatible-prefix
  contract with the full context the moment MakeMove/UnmakeMove will
  start touching it unconditionally (caught by this session's own
  TestSan self-test failure).

- Combining DEBUG=1 and TEST=1 for the first time (apparently ever)
  while chasing that failure immediately turned up three more real,
  pre-existing bugs, all in self-test-only code whose assert-guarded
  invariants had never been checked with asserts actually compiled in:
    - board.c: IsBoardLegal now rejects an en passant square whose rank
      doesn't match the side to move (and verifies the double-moved
      pawn/empty origin square), closing a gap VALID_EP_SQUARE alone
      can't catch.
    - testgenerate.c: TestCapturesAndPromotionsGenerator's fixture table
      had a self-contradictory FEN this new IsBoardLegal check would
      have caught; replaced with a genuine Black-to-move en passant
      position.
    - testgenerate.c: TestFindUnblockedSquaresSpeed called
      _GenerateAllMoves directly instead of the GenerateMoves wrapper,
      skipping the DEBUG-only pStack->board[uPly] snapshot
      WouldGiveCheck's own ASSERT checks against.

- precommit_check.sh / CLAUDE.md: added the DEBUG=1 TEST=1 combination
  as a permanent third pre-commit gate, given what its first-ever run
  turned up. Real cost (~235s of ~266s total, unoptimized build
  dominates), judged worth it.

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

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