Add a pure-sx match detector to the board model: `find_matches` walks each
row and column once in maximal same-type spans and marks every cell in a run
of length >= 3 into a `MatchMask` (a per-cell membership set mirroring
Board.cells). Overlapping shapes (L / T where a horizontal and vertical run
share a cell) collapse to the union automatically. `dump_matches` renders the
set deterministically: matched cells show their gem char, others '.'.
Detection only — no clear/collapse/refill (that is P2.1).
tests/match_detect.sx exercises hand-crafted boards (built explicitly on a
run-free checkerboard, no seeded init): a horizontal 3-run, a vertical 3-run,
multiple disjoint runs, length-4 and length-5 runs, intersecting L and T
shapes (shared cell counted once), and a no-match board. Output is locked as
tests/expected/match_detect.stdout (+ .exit) and asserts matched-cell counts.
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).
