refactor(game): extract undo.ts + drop undo as-any casts (Phase 2 of #132)

[nodots]

Phase 2 of the Game/index.ts decomposition epic (#133)

Stacked on #136 (Phase 1). Base is refactor/game-decomp-phase1-guards; retarget to development once #135 and #136 merge.

Depends on backgammon-types PR nodots/backgammon-types#68 — the undo.frames typing must land (and publish) for the cast removal to typecheck against the released types package.

What changed

1. Characterization tests first (test: expand core coverage, starting with Game/index.ts extraction targets #134) — 10 tests locking current behavior of Game.canUndoActivePlay / Game.undoLastInActivePlay (defensive/guard branches + a real-move push→pop happy path).
2. New src/Game/undo.ts — undoLastInActivePlay and canUndoActivePlay as free functions.
3. as any removal (chore: Eliminate as any casts in Game/index.ts with proper type definitions #96) — with undo.frames now typed BackgammonGameMoving[] on BasePlay, the functions read game.activePlay.undo.frames directly; 5 casts removed from the undo slice.
4. Public Game.* statics remain thin delegating wrappers. canUndoActivePlay (deferred from the guards phase) lands here with its concern.

Verification

• tsc -b --force clean
• npx jest: 458 passed / 12 skipped (448 baseline + 10 new)
• No behavior change; public Game.* surface unchanged

Refs #132, #96, #134