games_hub: chess, first variant king and pawn against king

[aaylward]

Adds chess as a game in games_hub, with one variant so far: king and pawn against king (kpk), starting from a random position.

• Rules engine (libs/chess_play): the start position plus the moves played, in UCI, over the vendored chess-library.
  • Mate, stalemate, insufficient material, the 50-move rule and threefold repetition all end the game automatically, with no claim needed.
  • Resignation ends it, and so does a player leaving (the leaver loses, and is credited a game played and not won).
• Clock: Fischer increment, running from the start of the game. A player who runs out of time loses, unless the opponent has only a bare king, in which case it's a draw.
  • A move that arrives after the mover's time ran out isn't played; the game ends on time instead. A leave after that point also ends on time.
  • A move's time is read under the hub lock, and no turn starts before the one it follows.
• Wire (model/chess.smithy): a chess envelope on the Play stream.
  • startGame sets the clock in seconds (30–1800 per side, 0–30 increment, default 3+2).
  • The view lists every legal move, so the client needs no chess rules of its own.
• Hub (golf_hub):
  • Chess tables seat 2.
  • SweepChessClocksOnce runs every 250 ms on each instance and ends games whose clock has run out. It goes through the same conditional commit as a move, so when several instances hold the room only one flag lands.
  • The sweep takes the lock once per game. If the store refuses a flag as unavailable, that game waits kChessFlagRetry (5 s) before the next attempt, so a database outage can't stall the hub on every tick.
  • SetClock and SetChessOpener let tests fix the time and the start position.
• Other places that list game names: chess is added to the prom_proxy tiles, the stats variant list and deja's VARIANTS.

What a reviewer won't see from the diff:

• TableEngineMove's commit loop is now ApplyTableEngineMoveLocked, so the sweep can reuse it without sending a rejection to a player who sent nothing. Castle and rummy go through it unchanged.
• bazel/3p/chess_library visibility now includes chess_play, whose headers expose no chess-library types.
• The stored row holds wall-clock epoch ms, so instances must agree on the time (NTP).

Tests:

• Engine and serde.
• A chess e2e suite with a fixed clock and start position. It covers every ending, the sweep, the clock thread, store-outage backoff, stats after a draw and after a leave, and the event-log lines.
• A raw-JSON wire test, which pins the decoder's bound refusals on a full table.
• The counter-vs-model pin.
• Postgres row and jsonb tests, plus a two-instance sweep that ends the game once.
• Go and Rust vocabulary tests.

Local runs:

• Every affected suite passed, including the Postgres-gated ones against a local PG 16 (they ran, not skipped).
• The chess e2e suite passed 15 runs.
• Hub and engine behaviors were mutation-checked.

Review panel: four read-only agents covering correctness, concurrency/storage, tests/docs, and altitude. Their fixes are in b1d0fc4.

Follow-ups:

• games_hub chess: bot opponents (stockfish, lc0, handrolled) #1618: bot opponents.
• games_hub: one adapter per game instead of per-game branches #1621: a per-game adapter in place of the per-game branches.
• make-git-overrides: cover the llvm toolchain's glibc source archive #1619: sandbox glibc override.

Not covered here:

• Draw offers.
• Lag compensation.

UI: muchq/muchq.github.io#394.

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[cursor]

Review panel — #1620 @ b1d0fc4c

Four lenses + Bugbot on the chess/kpk tip (incl. prior panel fold-ins). CI green. Bugbot clean. Altitude: kpk-first, legalMoves as accept-set, 250 ms sweep, and ApplyTableEngineMoveLocked are the right shapes. Clock/flag/leave/sweep races from the earlier panel look held.

Requested fold-ins

1. resign ignores an already-fallen flag — move and removePlayer both end on time first when remainingMs(side_to_move) == 0; resign does not. In the sweep gap, a flagged pawn side can resign into a loss and erase the bare-king timeout draw; the waiting side can resign into a loss instead of the flag win/draw. Mirror ALeaveAfterTheFlagIsTheFlag for resign. (line comment)

2. chess_flag_retry_at_ not erased when the game is already over — after an unavailable sets a key, a later ending elsewhere makes the scan hit isOver and continue without erase (:2818), and the re-lock path does the same (:2844). Small entries, but they live for the process. Erase on isOver in both places. (line comment)

3. restore accepts any player-ending Result on an open board — board endings are checked; resignation / timeout / abandoned are not. A hostile row can invent a timeout winner for the side that flagged, or a bare-king timeout win instead of a draw. Serde’s own contract says DB bytes are untrusted; the open-board resignation restore test currently pins the hole. At least refuse a timeout whose winner/nullopt disagrees with timeoutResult() at the stored clock, and refuse a resignation whose winner is the resigner. (optional harden; top-level)

Mild / optional

• README Observability still lists golf_* / castle_* / rummy_* / … without chess_* (hub emits chess_commands / chess_events; not in an active diff hunk for that bullet).
• Board-automatic ending spellings (stalemate / insufficientMaterial / fiftyMoves / repetition) are never string-pinned — only enums / player endings. A rename in EndingName keeps engine CI green and drifts wire + smithy together.
• Draw offers / lag are named “Not covered” with no durable follow-up (bots have #1618). File one ordered issue if you want the deferrals to survive the squash; for kpk, draw offers outrank lag.

Checked and not raised

Fischer increment, move-after-expiry, bare-king timeout draw, leave-after-flag, sweep vs move conditional commit / two-instance pin, NowMs under lock, turn_started skew clamp, unavailable backoff, castle/rummy through ApplyTableEngineMoveLocked, BUILD srcs, clock bounds wire goldens, stats/prom_proxy/deja/otel pins.

[aaylward]

Addressed in 024443f:

• Resign after the flag, and the retry-map leak: fixed; see the replies on the line comments.
• Timeout in a stored row: restore now refuses a timeout the stored clock contradicts: time still left on the side to move, or a result other than the one a flag gives (RestoreRefusesATimeoutTheClockContradicts). The resignation half can't be checked, because the row doesn't record who resigned, and either color can win by resignation. That stays accepted.
• README: Observability now lists chess_*.
• Ending spellings: all eight are pinned both ways (EveryEndingIsSpelledAsTheWireSaysIt).
• Draw offers and lag: filed as games_hub chess: draw offers, then lag compensation #1622, in that order.

---

Generated by Claude Code

[cursor]

Recheck @ 024443f: panel items closed.

• Resign after flag → timeout first; AResignationAfterTheFlagIsTheFlag pins both seats + control.
• Flag-retry map: erase on isOver at re-lock, plus per-sweep erase_if for gone/over games; AGameThatEndsWhileItsFlagWaitsLeavesNoRetryBehind.
• restore refuses a timeout the clock contradicts; resignation left open (no resigner on the row) — agreed.
• README Observability lists chess_*; all eight EndingName spellings pinned both ways; draw offers / lag filed as games_hub chess: draw offers, then lag compensation #1622.

Nothing further from the panel. Watching CI on this tip.

[cursor]

Recheck @ 01d7d0c5: leaver stats credit both seats from state.players() (chess keeps them; no engine compact), so the leaver gets played+loss and the stayer the win. ALeaverLosesByAbandonment pins (1,0) / (1,1). Matches the README. No new panel findings.