refactor(game): remove as-any casts in cube + snapshot paths (Phase 9 of #96)

[nodots]

Phase 9 of the epic (#133) — as any removal (#96)

Stacked on #143 (Phase 8). Retarget to development as parents merge.

Continues the #96 cast removal beyond the undo slice (done in Phase 2).

cube.ts — all 5 as any removed

• offeredThisTurnBy is declared on BaseCube, so (game.cube as any).offeredThisTurnBy was unnecessary.
• rollForStartValue is on BasePlayerProps (optional) but required on the rolling/inactive player types → the two player-copy casts become documented non-null assertions.
• The acceptDouble return as any narrows to a commented as unknown as BackgammonGameRolling (the spread yields a generic players array TS can't narrow to the tuple shape).

turnFlow.ts — undo-snapshot casts removed

game.activePlay and undo.frames are typed now (since the Phase 2 types fix), so the const ap: any = (game as any).activePlay blocks in move() and executeAndRecalculate() — plus their JSON-fallback as any — are gone, replaced by a typed ap.undo ?? (ap.undo = { frames: [] }).

index.ts

The gnuPositionId getter's this as any now carries the required explanatory comment (Game class vs BackgammonGame union).

Deferred

turnFlow.ts still has ~30 (move as any).x = casts in the sanitization/recalculation loops. Removing them means rebuilding move objects immutably instead of mutating in place — a behavior-sensitive change in the hottest game path, left for a dedicated pass now that turnFlow has ~76% coverage.

Verification

• tsc -b --force clean
• npx jest: 518 passed / 12 skipped, unchanged

Refs #96