P3.2: cascade combo multiplier (pure sx)

Scale each cascade round's base points by combo_multiplier(round) = the
1-based round index (round 1 x1, round 2 x2, ...), so deeper chains pay
out more. resolve now reads score_round before each clear, accumulates
score_round * combo_multiplier(round) into Board.score, and reports the
settle's payout as the new Cascade.awarded field. A depth-1 settle scores
exactly its base (x1, no bonus); any multi-round chain strictly exceeds
the same clears scored flat.

resolve_step keeps its signature (no scoring), so cascade.sx and its
golden are unchanged; score_round/add_round_score are untouched, so
score.sx is unchanged. New tests/combo.sx golden locks exact cumulative
scores for a single-round clear (30), the P2.4 cascade board (flat 60 ->
mult 90), and a controlled 3-round chain (flat 90 -> mult 180), printing
per-round base/multiplier/points so the golden self-explains.

tests/expected/combo.exit

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+0

tests/expected/combo.stdout

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+== combo (cascade multiplier) ==
+== single-depth1 ==
+RRRGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+OGOGOGOG
+GOGOGOGO
+round 1: cleared 3 base 30 x1 = 30
+flat sum 30
+multiplied total 30
+== cascade-depth2 ==
+OGOGOGOG
+GOGOGOGO
+RGOGOGOG
+BBBOGOGO
+RGOGOGOG
+ROGOGOGO
+OGOGOGOG
+GOGOGOGO
+round 1: cleared 3 base 30 x1 = 30
+round 2: cleared 3 base 30 x2 = 60
+flat sum 60
+multiplied total 90
+== chain-depth3 ==
+OGOGOGOG
+GOGOGOGO
+RGOGOGOG
+BBBOGOGO
+RGOGOGOG
+ROGOGOGO
+OGOGOGOG
+GOGOGOGO
+round 1: cleared 3 base 30 x1 = 30
+round 2: cleared 3 base 30 x2 = 60
+round 3: cleared 3 base 30 x3 = 90
+flat sum 90
+multiplied total 180
+ok: combo multiplier scales cascade rounds