P2.4: cascade resolution loop (pure sx)

Add the settle loop a swap triggers: resolve(board) runs rounds of
detect -> clear -> collapse -> refill until a round finds no match,
returning a Cascade { depth, cleared } so P3 can read per-round
cleared-cell counts and the combo-driving depth. resolve_step exposes
one round. Termination follows from eventually reaching no-match; no
artificial round cap.

tests/cascade.sx: a fixed-seed hand-crafted board where clearing the
initial BBB lets gravity pack col 0 into a fresh vertical RRR, so the
loop chains two rounds; snapshots each per-round board and the final
depth, asserts the final board is stable and that public resolve
reproduces the manual loop, plus a depth-0 control on an unchanged
checkerboard. Locked in tests/expected/cascade.{stdout,exit}.

tests/expected/cascade.exit

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

tests/expected/cascade.stdout

@@ -0,0 +1,30 @@
+== cascade (resolution loop) ==
+start:
+OGOGOGOG
+GOGOGOGO
+RGOGOGOG
+BBBOGOGO
+RGOGOGOG
+ROGOGOGO
+OGOGOGOG
+GOGOGOGO
+round 1: cleared 3 cells
+RBRGOGOG
+OGOOGOGO
+GOGGOGOG
+RGOOGOGO
+RGOGOGOG
+ROGOGOGO
+OGOGOGOG
+GOGOGOGO
+round 2: cleared 3 cells
+RBRGOGOG
+PGOOGOGO
+YOGGOGOG
+RGOOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+cascade depth 2
+ok: cascade resolves to a stable board