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.
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