Add board.sx, the headless Phase-1 match-3 core:
- Gem enum (6 types, ordinal 0..5) + single-char dump alphabet.
- Rng: a 32-bit LCG carried in s64, masked to 32 bits each step, so the
stream is host-width independent and valid for any seed.
- Board (8x8, row-major) with idx/at/set accessors and a seeded init that
fills row-major, excluding any gem that would complete a 3-in-a-row with
the two cells to the left or above — so the result has zero pre-existing
matches. Single RNG draw per cell, always terminates.
- board_dump: deterministic one-row-per-line textual snapshot.
tests/board_init.sx seeds with a fixed seed, dumps the board, and asserts
zero horizontal/vertical 3-in-a-row runs via an independent scan. Output and
exit code are locked as goldens. App ios-sim build is unaffected (main.sx
does not import the model yet).
