UCI: stop FEN parsing at the "moves" keyword

[testtest126]

Summary

The UCI adapter's position fen <fen> moves ... handler collected FEN fields with a blind prefix(6).

Board(fen:) accepts a short FEN (4–5 fields — it defaults the halfmove/fullmove clocks, Board.swift:24 parts.count >= 4). But the blind take doesn't stop at the moves keyword, so for a short FEN it pulls moves and the move tokens into the FEN string. The FEN still parses (the non-integer trailing fields just default the clocks), but index then lands past the moves keyword — so the trailing moves are silently dropped and only the base FEN position is applied.

Fix: collect FEN fields up to the moves keyword (still capped at six). Full 6-field FENs are unaffected (the sixth field is followed by moves, so the cap and the keyword-stop agree); short FENs now keep their moves.

Scope / gates

• Security-sensitive: no — a pure input parser; no auth/token/crypto/session code.
• App-touching: no — ChessProtocol package only. UCIEngine is public API of that module but isn't wired into the iOS app or server today, so this is a correctness fix for the adapter's public contract rather than a user-facing change.
• Shared files: none.

Test plan

New testPositionShortFENWithMoves in UCITests: position fen 4k3/8/8/8/8/8/4P3/4K3 w - - moves e2e4 (a 4-field FEN + a move) must land the pawn on e4 with e2 empty. Pre-fix the move is dropped (pawn stays on e2). Existing testPositionFEN / testPositionFENWithMoves (full 6-field FENs) guard against regression.

CI is the verifier — this session can't build the Swift toolchain locally (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)

---

Generated by Claude Code

[testtest126]

@orchestrator — merge request. This PR is fully CI-green with auto-merge armed, 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 (pure UCI input parser), no shared files, review threads resolved. Batched with #161 and #177 — see the request on #161 for the full set. Requesting approval + merge.

---

Generated by Claude Code