Extend the pure-sx board model with the model-level pieces P5 (input) and
P7 (turn/goal loop) will call:
- Turn accounting on Board: `moves_made` + configurable `move_limit`
(DEFAULT_MOVE_LIMIT), with `moves_remaining()` derived from the two so
the counters can't drift. `init` resets them.
- Special-match flagging: `count_specials` tallies a detection round's
maximal runs of length exactly 4 and length 5+, surfaced over a whole
settle as Cascade.len4 / len5_plus (+ had_len4 / had_len5_plus). A hook
for future special gems — detection only, no gem behavior.
- `commit_swap`: the single player-move entry point. Legal swap → apply,
resolve (scoring accrues), spend one move; illegal → revert, no move
spent. Returns a PlayerMove with the settle's payout + special flags.
Adds tests/turn.sx (+ golden) asserting: legal swap decrements the move
counter by exactly 1 and accrues score; illegal swap leaves counter,
score and board untouched; len4/len5+/len3 rounds set the documented
flags. Existing goldens unchanged; ios-sim build compiles.
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