P5.2: swipe commits legal swap / reverts illegal (sx, iOS sim)

Wire touch input into the model in BoardView.handle_event. A press records
the drag start (new DragInput, heap-allocated so it survives the per-frame
BoardView rebuild between mouse_down and mouse_up); the release resolves the
gesture against the same layout it was drawn with. A swipe — start→end mapped
by swipe_intent to an adjacent-swap intent — is fed straight into
commit_swap: a legal swap applies, cascades (clear→collapse→refill), accrues
score and spends a move; an illegal one reverts, no move. A sub-threshold /
off-board drag carries no intent and falls back to the tap behaviour
(toggle/clear selection). The next frame re-renders board + HUD from the model.

Reuses swipe.sx + board_layout.sx + commit_swap unchanged — this is wiring,
not new legality/cascade logic.

tests/swipe_commit.sx (new golden) drives the full path on the seeded board
(SEED 1337): a rightward swipe (0,0)->(1,0) is illegal (two reds) and reverts
byte-for-byte with no score/move; (5,4)->(6,4) is legal, completes R,R,R on
row 4, awards 30, spends one move.

Sim evidence (iPhone 17, iOS 26.0): goldens/p5_swap_before.png (SCORE 0,
MOVES 30/30) and goldens/p5_swap_after.png (SCORE 30, MOVES 29/30) bracket a
real idb-injected swipe at (276,475)->(327,475) pt = cell (5,4)->(6,4); the
three reds clear and the board matches the model's resolved state.

tests/expected/swipe_commit.stdout

@@ -0,0 +1,24 @@
+== illegal swipe reverts ==
+RRPPOGRG
+PGPOPRRO
+YYBBYRYB
+GBYYRGGP
+OGBRRORY
+BYRRPRBG
+YOYYROBB
+OROBPPRB
+intent (0,0)->(1,0)
+legal false awarded 0 score 0 moves_made 0 moves_remaining 30
+== legal swipe commits ==
+intent (5,4)->(6,4)
+legal true depth 1 awarded 30 score 30 moves_made 1 moves_remaining 29
+after:
+RRPBORRG
+PGPPOGRO
+YYBOPRYB
+GBYBYRGP
+OGBYRGOY
+BYRRPRBG
+YOYYROBB
+OROBPPRB
+ok: swipe reverts illegal, commits legal