Engine: detect draw by repetition within the search

[testtest126]

Summary

NegamaxEngine recognized checkmate, stalemate, the fifty-move rule, and insufficient material as draws — but not a position recurring on the search line. Consequences:

• In a won position it could value a repeating line as still winning and shuffle into a draw of a game it was winning (the classic "engine draws a won game").
• In a lost position it could not steer into a saving perpetual, because the repeating line was scored by material, not as the draw it is.

Fix: track the Zobrist key of each position on the current line, indexed by ply, and score a node whose position already occurs among its same-parity ancestors (a position recurs only with the same side to move) as a draw — checked before the transposition table, whose score is path-agnostic. It reuses the key already computed for the TT probe, so the hot path only pays an ancestor scan. Quiescence is deliberately skipped: its moves are captures and promotions, both irreversible, so no position can recur there. PersistentNegamaxEngine builds a fresh Search per call, so nothing leaks across searches.

On the bench signature (from #159)

The determinism signature pinned in EngineLabTests/BenchTests.swift is unchanged: none of the 20 bench positions produces an in-search repetition at the CI node budget, so search behavior on that suite is identical. (Repetitions are rare in short searches from fresh roots — they need a back-and-forth line, which the bench positions don't reach at 2000 nodes.) Behavioral coverage of the new logic therefore comes from the dedicated test below, not the bench signature.

Scope / gates

• Security-sensitive: no — engine search only; no auth/token/crypto/session code.
• App-touching: no — ChessProtocol package; iOS lane N/A. (The app's engine benefits, but no app code changes.)
• Shared files: none — does not touch ChessOnline/Messages.swift.

Test plan

New testRecognizesPerpetualCheckAsDraw in EngineTests uses a queen endgame independently confirmed drawn (verified with an external engine at both shallow and deep search): White is down a pawn with Black's a2-pawn a step from promoting, held only by a forced perpetual 1.Qd3+ Kb4 2.Qd4+ Kb5 3.Qd3+ …. Without repetition awareness the search scores the line by material (a lost pawn, soon a new black queen) and reports White as worse; with the fix the repeating line is a draw — the best White has — so the search returns exactly 0. Existing testFindsMateInOne / testCapturesFreeQueen / testSearchIsDeterministic guard that normal search is unchanged.

CI is the verifier here (this session can't build the Swift toolchain locally — the proxy blocks the download):

• swift test in ChessKit/ passes — via the ChessKit tests CI lane
• swift test in chess-server/ passes — N/A (no server change)
• App builds / relevant UI tests pass — N/A (no app change)

[testtest126]

@orchestrator — merge request. This PR is fully CI-green and ready-for-review, stuck only on the orchestrator-approval status (only you can post it; nothing has merged repo-wide since ~2026-07-12 20:42 UTC). Non-security (engine search only — no auth/token/crypto/session code), does not touch ChessOnline/Messages.swift, review threads resolved. Batched with #161 and #164 — see the request on #161 for the full set. Requesting approval + merge.

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