Add Canfield game variant

[austin-smith]

Closes #51.

What Changed

• Adds Canfield as the tenth game variant, implementing the strict classic rules: a thirteen-card reserve whose exposed card is always playable and which must fill any empty tableau pile at once; four foundations built up by suit from a dealt base rank, wrapping King to Ace; a four-pile tableau built down in alternating colors, wrapping Ace to King, with whole-pile moves only; a draw-three stock with unlimited redeals; and locked foundations. Spaces fill only from the waste once the reserve is out, and the exposed waste top stays playable after a spent fan.
• GameState gains a reserve pile (back-compatible decoding) and Selection.Source a .reserve case; the shared foundation rule gains a state-aware path for Canfield's base-rank starts. Every other variant's persistence rules now reject cards stranded in the reserve field, mirroring the existing foreign-field guards.
• Hints come from CanfieldPlanner, a bounded best-first search over moves and stock taps with a recycle-aware position hash, followed as cached improving lines. Because a tap on the spent stock recycles, an exhausted search is a proof the game is over, so hints go silent there instead of churning a dead stock; only truncated verdicts fall back to a tap.
• The compulsory reserve fill scores like any reserve-to-tableau play, matching how Klondike's compulsory flip scores through its automatic path.
• Moves recycleWaste from the Klondike session file to the shared stock section now that two variants recycle, and fixes a macOS Game menu crash assembling a Character("10") keyboard shortcut for a tenth variant (⌘1–⌘9, then ⌘0).
• Rules doc (docs/rules/canfield.md), README rows, in-app rules/terms/scoring, statistics, screenshot fixture, and the hint-probe wiring and ledger row.

Why

Canfield (Demon in Britain) is a classic reserve-based patience the app was missing (#51). The implementation follows the printed classic sources over the relaxations common in software — whole-pile moves only, no worrying back, compulsory reserve fills — with the deliberate choices documented in the rules doc.

Validation

• Full test suite passes on macOS, including four new Canfield suites (rules wrap edges, whole-pile pickup, space-fill gating, reserve refill and scoring, recycling, planner line following, persistence corruption) and a shared test proving every other variant rejects reserve-stranded cards.
• Hint probe, 500 seeded deals: 25.0% following every hint vs 1.2% random control, zero stalemate loops, zero position revisits (gated), zero action caps — every loss an exhaustively-proven deadlock. A full all 500 study reproduces every existing variant's ledger figure exactly, so the shared-code changes are behavior-preserving outside Canfield.
• Verified in the iOS simulator: deal layout, draw-three fan, reserve tap auto-move with reveal and count, base-rank foundation placeholders, and the game-mode picker thumbnail.

UI Changes

• New Canfield board: Klondike-shaped top row (stock, fanned waste, four foundations) with the reserve in the tableau band at the left column, its count in felt-set type below, and the four piles aligned under the foundations.
• Empty foundation placeholders show the dealt base rank in Canfield instead of the Ace glyph.
• Game-mode picker gains a Canfield row with a mini-board silhouette; rules/terms/scoring and statistics screens gain Canfield content.