Land bitboard move generation (Part A+B) and movesup.c bitboard queries; default on

Implements the full board_representation/MOVEGEN_MIGRATION.md scope:
bitboard-backed generators for all six not-in-check piece types plus
the JumpTable-avoiding whole-node dispatch fork (_GenerateAllMovesBB),
the in-check escape path (king flight + block/capture), and
movesup.c's ExposesCheck/FasterExposesCheck/ExposesCheckEp/IsAttacked/
InCheck bitboard equivalents. Nine toggles total
(GENERATE_{KNIGHT,KING,ROOK,BISHOP,QUEEN,PAWN}_BITBOARD,
GENERATE_ESCAPES_{KING,BLOCK}_BITBOARD, EXPOSESCHECK_BITBOARD,
ISATTACKED_BITBOARD), all now on by default in GNUmakefile --
DISABLE_BITBOARD_MOVEGEN=1 opts back into the mailbox path, which
remains fully present and compiled either way.

Correctness verified via perft (Kiwipete, Position 4), the move-set
comparison harness across 20,000 random positions, all nine toggles
combined cleanly (15/15 runs, after fixing a GenerateRandomLegalPosition
en-passant-sentinel bug in the test harness), and sd10 on all three
curated suites showing zero solve-count regression vs head_reference
(the ecm_hard_quick delta traced to unrelated intervening commits).
Speed: most individual generators land near parity by design (mailbox's
per-square walk was already close to O(destination count)); the real,
consistent wins are the dispatch-layer fork (up to 23% in dense
positions) and IsAttackedBB (0.73x-0.93x of mailbox).

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01AbHkVrm5KUyzLwWd3GHmo6

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