Add Yukon game variant

[austin-smith]

Closes #23.

What Changed

Adds Yukon as a third game variant alongside Klondike and FreeCell, with the same first-class treatment: solver-backed hints, persistence, statistics, scoring, auto-finish, accessibility, tests, rules documentation, and a marketing screenshot fixture.

Rules (verified against the three sources linked in #23 — Wikipedia, cardgames.io, 247solitaire, which agree):

• Seven piles, all 52 cards dealt (1 face-up; then n−1 face-down under 5 face-up per pile); no stock or waste.
• Klondike landing rules (build down, alternating colors; Kings only on empty piles), but any face-up card moves together with every card stacked on it, regardless of order — only the moving group's bottom card must fit.
• Foundations by suit Ace→King; exposed face-down cards flip; Klondike-style scoring plus the win time bonus.

Architecture mirrors the existing per-variant structure (Game/Yukon/, Views/Yukon/, ComputerSolitaireTests/Yukon/). The one shared-core seam is a fifth AutoMoveAdvisor variant hook, variantAllowsTableauPickup (Klondike/FreeCell delegate to the existing sequence check verbatim). Where variants genuinely converged, logic was consolidated instead of copied: shared BinaryHeap (first commit, mechanical), king-anchored landing rule, face-down tap/flip handlers, stockless session configuration, advisor king-transfer/flip helpers, and GameState.isWon.

Hint system (YukonPlanner + cached lines in HintPlanner):

• Bounded best-first search; HintPlanner caches each improving line keyed by position (FreeCell's pattern) because Yukon's reversible moves make per-move re-search oscillate.
• Staged move generation: a rollback-free search first; the full move set including foundation→tableau rollbacks only when the first stage exhausts its graph — so rollback-only rescues are found, and a nil hint is an honest verdict, not a guess.
• Hints are strict: every hint is the first move of a verified improving line, or silence (FreeCell keeps its nudge fallback deliberately — its solver misses are usually winnable positions; Yukon's are not, and the asymmetry is documented in HintPlanner).

Statistics: GameStatistics gains a backward-compatible variant-neutral highScore for stockless variants (FreeCell/Yukon); Klondike keeps its per-draw-mode fields, and the stats screen now only shows draw-mode rows on the Klondike tab.

Why

Yukon plays like Klondike's wilder sibling and was the requested next variant (#23). The hint system follows the repo's solver-backed standard rather than heuristics, per the measured-baselines tradition.

UI Changes

New Yukon board (top row is four foundations over tableau columns 4–7; no stock/waste), a Yukon card in Settings, Yukon arms in Rules & Scoring, a Yukon statistics scope, and a single "High Score" row replacing the two Klondike draw-mode rows on FreeCell/Yukon stats tabs.

Validation

• Full unit suite passes (macOS); iOS and macOS builds clean. 24 new/extended tests: deal shape, landing/pickup rule matrix, unordered group moves through the session, same-pile drop rejection, flip-on-expose, scoring deltas, persistence round-trip and layout rejection (incl. stranded free-cell cards), auto-finish candidacy/simulation, planner determinism, loop-freedom of cached lines, rollback-rescue and rollback-only-position hints, truncation vs. exhaustion semantics, deadlock verdicts, and an end-to-end hint-following win on a probe-verified seed.
• Release-build probe harness (repo tradition, out-of-tree), 500 seeded deals: 61.8% hint-following win rate vs 0% for both greedy-tap and random baselines (theoretical winnability ≈80%); zero hint loops, zero exact-state revisits; p95 hint latency ≈0ms via cached lines (max ~380ms one-off at the search deadline).
• Klondike/FreeCell regression probes are bit-identical pre/post heap migration and shared-code consolidation (FreeCell 97/100 solves, Klondike draw-1 41/100 on fixed probe seeds).
• Manual simulator check of the staged Yukon board (deal shape, foundation alignment, HUD).

Note: probe harnesses are intentionally not committed, matching the existing hint-quality-baseline workflow; figures above are reproducible from GameStateFixtures.seededYukonDeal.