Add Forty Thieves game variant

[austin-smith]

What Changed

• Adds Forty Thieves: two decks dealt into ten columns of four face-up cards, tableau built down by suit with strictly single-card moves, eight player-built foundations that lock once a card lands, and a draw-one stock with a single pass and no recycle. Rules follow Add Forty Thieves game variant #49 exactly, verified against all three of its references; deliberate rule choices are documented in docs/rules/fortythieves.md.
• Solver-backed hints via a new FortyThievesPlanner — a Spider-family cached improving-line best-first search with two-deck-aware canonicalization (twin-top and same-suit-foundation dedup, first-empty-column) and a monotone stock-tap fallback that structurally cannot loop. Wired into HintPlanner with a 0.3s interactive budget.
• New GameVariant.allowsFoundationRollback separates locked foundations from Klondike-style rollback foundations; the tap policy gains a Forty Thieves-specific safe-banking rule (rank r is safe once both same-suit foundations reach r − 2), and auto-finish fires once the stock is empty, playing tableau tops and the waste.
• Full variant surface: top-row and mini-board views, in-app rules/scoring/terms, README rows, generated screenshot fixture, hint-probe wiring with a recorded ledger baseline, and per-variant test suites.
• Fixes two review findings in the shared tableau deal flight (Spider/Scorpion): a gameplay move landing mid-flight now lands the flight instead of finishing against a stale frame (mirroring the undo path's existing rule), and the pre-flight frame wait no longer stalls 240ms when a dealt card completes a run and banks on arrival.

Why

Closes #49. Forty Thieves is the best-known two-deck patience and builds on the two-deck support introduced for Spider, while being the app's first variant to combine two decks with player-built foundations — hence the new rollback distinction rather than overloading playerBuildsFoundations.

Validation

• 493 unit tests pass on macOS (43 new across Forty Thieves rules/session/persistence/planner suites); iOS Simulator build green.
• Hint probe, 500 seeded deals: fortythieves 3.4% hint-following vs 0.0% random control, every loss an honest deadlock, zero stalemate loops, zero revisit events (now gated like Yukon's); baseline and measured tuning directions recorded in the ledger. The scorpion baseline reproduces exactly post-merge (14.8% / 2.8%), confirming the sync changed nothing behaviorally.
• Played end-to-end in the iOS simulator: deal shape, same-suit builds, empty-column fills, waste plays, foundation banking, locked foundations, no stock recycle, hint highlighting, and the mode picker; Scorpion's three-card deal flight re-verified after the animation fixes.

UI Changes

• New Forty Thieves board: stock and waste on the left with eight foundation slots aligned 1:1 over the ten tableau columns, plus a matching mode-picker mini-board slotted between Golf and Yukon.
• New in-app Rules & Scoring content for the variant.