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feat(presets): berolina-pawns en-passant (Parton 1952, both variants)

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Feature 3 of post-epic-deferrals. Adds en-passant to berolina-pawns
and berolina-pawns-2 following the Parton 1952 variant — the most
common published ruleset. Reflects standard ep semantics through
Berolina's reversed geometry (diagonal push, orthogonal capture).

Engine:
  - MoveHookContext extended with pieceId + from + to. Existing
    presets (piece-hp, poisoned-squares, etc.) are purely additive
    on the new fields and don't need changes. Dispatch site in
    applyMove populates them from the resolved move.

berolina-pawns + berolina-pawns-2:
  - overridePieceMoves: after emitting normal Berolina moves, read
    the preset's ep latch (namespaced preset state). If the mover's
    orthogonal-forward square equals the stored skipped square AND
    the capturer color matches, emit a capture move onto that
    square. Note destination is empty by construction — the engine's
    getPieceAt returns null there, so the default capture path is a
    no-op; onAfterMove below does the actual retraction.
  - onAfterMove: two responsibilities. (1) If the mover just
    accepted a latched ep (move.to === skippedSquare) retract the
    stored capturedPieceId. (2) Clear the latch. (3) If THIS move
    was a Berolina double-diagonal push from the home rank, record
    skippedSquare + capturedPieceId (the mover's own pieceId, since
    that's what the opponent can remove next turn) + capturer color.
  - Scope-flip semantics preserved — scope='white' means only white
    pawns emit ep moves; the latch still records for downstream, but
    the opposing black pawns follow FIDE rules and don't emit it.
  - berolina-pawns-2: sideways captures do NOT trigger or accept
    ep (only the orthogonal-forward direction participates).

Tests:
  - 5 new berolina-pawns.test.ts cases: (r) latch + ep emission,
    (s) accept-ep retracts the double-pushed pawn, (t) one-half-move
    window expiry, (u) single-push doesn't latch, (v) scope='white'
    latch set on white's double-push.
  - 3 new berolina-pawns-2.test.ts cases: (l) ep through the
    extended preset, (m) retraction on accept, (n) sideways capture
    does NOT set the latch.
  - All 31 pre-existing Berolina tests unchanged and still pass.

Docs:
  - RULES.md gallery entries: remove 'en-passant deferred' language;
    document the Parton 1952 rule and the sideways-ep exclusion.
  - PRESET-API.md post-landing-backlog: drop the berolina ep
    deferral.
  - Preset docblocks: rewrite the en-passant section to describe
    the shipped mechanism + plan reference.

Verification: 1671 unit tests (+8 ep). Typecheck + lint clean.

Plan: .sisyphus/plans/post-epic-deferrals.md Feature 3 complete.
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Joey Yakimowich-Payne 2026-04-21 11:06:53 -06:00
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packages/chess/RULES.md
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@ -461,15 +461,15 @@ Win by wiping out every enemy piece of a specific TYPE (default: `"pawn"`). Targ
### Berolina Pawns
**ID**: `berolina-pawns` · **Scope-aware**: YES · **Mode**: hook (overridePieceMoves)
**ID**: `berolina-pawns` · **Scope-aware**: YES · **Mode**: hooks (overridePieceMoves + onAfterMove)
Pawns push forward DIAGONALLY and capture forward ORTHOGONALLY (inverse of FIDE pawns). Diagonal double-push from home rank, promotions on either push or capture reaching last rank. En-passant NOT implemented (deferred; document in future). Scope chooses which side(s) use the rule: `scope: "white"` only white pawns are berolina, `scope: "black"` only black, `scope: "both"` both sides.
Pawns push forward DIAGONALLY and capture forward ORTHOGONALLY (inverse of FIDE pawns). Diagonal double-push from home rank, promotions on either push or capture reaching last rank. En-passant follows Parton 1952 semantics (post-epic Feature 3): a double-diagonal push passes through an intermediate square; the opponent may capture onto that square on the very next half-move via their orthogonal-forward capture vector, retracting the double-pushed pawn. Sideways captures are not available on this preset (see `berolina-pawns-2`). Scope chooses which side(s) use the rule: `scope: "white"` only white pawns are berolina, `scope: "black"` only black, `scope: "both"` both sides.
### Berolina Pawns (Extended)
**ID**: `berolina-pawns-2` · **Scope-aware**: YES · **Mode**: hook (overridePieceMoves)
**ID**: `berolina-pawns-2` · **Scope-aware**: YES · **Mode**: hooks (overridePieceMoves + onAfterMove)
Like `berolina-pawns` but ALSO allows SIDEWAYS captures (adjacent file, same rank). Same scope-flip semantics.
Like `berolina-pawns` but ALSO allows SIDEWAYS captures (adjacent file, same rank). Same scope-flip semantics and the same Parton 1952 ep rule (orthogonal-forward ep only — sideways captures do not trigger or accept ep).
### Bouncing Pieces