Add chess

The full game: castling with every condition, en passant including the
case where taking would expose your own king, promotion to any of the
four, pins, checkmate, stalemate, insufficient material, the fifty-move
rule and threefold repetition.

Move generation is checked against the published perft counts rather
than against hand-written cases — the opening array to depth four
(197,281 positions) plus four standard test positions chosen precisely
because they break generators. Those counts are exact, so matching them
leaves very little room for a rule to be subtly wrong.

The opponent is a negamax search with alpha-beta, iterative deepening
and a capture-only quiescence tail, at three strengths. The quiescence
tail is the part that stops it hanging pieces: a fixed-depth search that
stops mid-exchange scores the position as though the recapture never
comes.

Two supporting changes:

The AI now thinks off the caller's thread. A card AI answers instantly,
but a chess search runs for a second or more, and the match controller's
scope belongs to the screen — searching there would freeze the board it
is drawn on.

Offline setup can seat the local player somewhere other than seat 0,
which is what lets you play Black. Seat 0 is White by definition, so
choosing a colour has to choose a seat.

FEN is read and written, so a table can start from a set position.

203 engine tests pass locally.

README.md

@@ -5,12 +5,12 @@ or against other people over Wi-Fi (LAN) or Bluetooth.
 
 ## What works today
 
-The app shell is complete and seven games are playable end to end:
+The app shell is complete and eight games are playable end to end:
 
 | | |
 | --- | --- |
-| **Playable** | Euchre, Kaiser, President, Golf, Wizard, Crazy 8s, Sequence |
-| **Listed, not yet implemented** | Mastermind, Backgammon, Chess, Checkers |
+| **Playable** | Euchre, Kaiser, President, Golf, Wizard, Crazy 8s, Sequence, Chess |
+| **Listed, not yet implemented** | Mastermind, Backgammon, Checkers |
 
 Unimplemented games appear in the menu marked *Coming soon* **on dev builds
 only** — a production release lists just the games that can actually be played,
@@ -153,6 +153,7 @@ game:
 - [Golf](docs/RULES-golf.md)
 - [Wizard](docs/RULES-wizard.md)
 - [Crazy 8s](docs/RULES-crazy8s.md)
+- [Chess](docs/RULES-chess.md)
 
 ## Layout
 
@@ -167,6 +168,7 @@ game/          rules engines and AI — pure Kotlin, no Android, unit-tested
   golf/        Golf model, scoring, rules, AI
   wizard/      Wizard model, rules, AI
   crazyeights/ Crazy 8s model, rules, AI
+  chess/       Chess model, FEN, move generation, search
 net/           wire protocol, LAN and Bluetooth transports, lobby, match driver
 settings/      DataStore-backed preferences
 update/        GitHub Releases OTA updater