Add swap + legality to the board model:
- swap(board, a, b): in-place, self-inverse cell exchange (trial then revert).
- adjacent(a, b): orthogonal-adjacency predicate (diagonal/gap = false).
- swap_legal(board, a, b): legal iff adjacent AND, after the trial swap, either
swapped cell participates in a 3+ match (reuses find_matches); leaves the
board unchanged. Non-adjacent/diagonal rejected before any match check.
- Cell/Swap structs + legal_swaps(board): all currently-legal swaps in a stable
row-major, right-before-down order; dump_swaps for deterministic snapshotting.
tests/swap_legality.sx asserts the predicate over hand-crafted boards (legal
3-run, no-match, non-adjacent, diagonal, only-the-other-gem-matches) and the
non-mutating revert; locks legal_swaps over the seeded board as a golden.
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