P3.3: turn accounting + special-match flagging (pure sx)

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.

tests/expected/turn.exit

@@ -0,0 +1 @@
+0

tests/expected/turn.stdout

@@ -0,0 +1,98 @@
+== turn (accounting + special-match flagging) ==
+== flag-len3-none ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GORRROGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+len4 0 len5_plus 0
+had_len4 false had_len5_plus false
+== flag-len4 ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GORRRRGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+len4 1 len5_plus 0
+had_len4 true had_len5_plus false
+== flag-len5 ==
+OGOGOGOG
+GRRRRRGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+len4 0 len5_plus 1
+had_len4 false had_len5_plus true
+== flag-len4-and-len5 ==
+RRRGOGOG
+GOGOGOGO
+GOBBBBOG
+OGOGOGOG
+GPPPPPGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+len4 1 len5_plus 1
+had_len4 true had_len5_plus true
+== commit-legal-len3 ==
+RROGOGOG
+GGROGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+before: score 0 moves_made 0 moves_remaining 5
+after: legal true depth 1 awarded 30 len4 0 len5_plus 0
+after: score 30 moves_made 1 moves_remaining 4
+RBRGOGOG
+GGOOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+== commit-illegal ==
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+before: score 0 moves_made 0 moves_remaining 5
+after: legal false depth 0 awarded 0 len4 0 len5_plus 0
+after: score 0 moves_made 0 moves_remaining 5
+== commit-legal-len4 ==
+RROROGOG
+GGROGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+after: legal true depth 1 awarded 60 len4 1 len5_plus 0
+after: had_len4 true had_len5_plus false
+after: score 60 moves_made 1 moves_remaining 4
+RBRROGOG
+GGOOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+ok: turn accounting + special-match flagging