docs: add currying public API design spec

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/superpowers/specs/2026-03-31-currying-public-api-design.md

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+# Currying Public API — Design Spec
+**Branch:** feat/NCS-11
+**Date:** 2026-03-31
+
+---
+
+## Overview
+
+Refactor the public methods of `MoveValidator` and `GameRules` to use multiple parameter groups (currying), separating `(board[, history])` as the context group from the computation parameters. This is a pure style refactoring — no behaviour changes, no new tests required.
+
+---
+
+## Motivation
+
+Currying clarifies intent: `(board, history)` is "the world being operated on"; the remaining parameters are "what varies." It also enables partial application at call sites where board/history are fixed across a loop (e.g. `isInCheck`, `legalMoves`).
+
+---
+
+## Section 1: Methods Being Curried
+
+### `MoveValidator` (public API only)
+
+| Before | After |
+|--------|-------|
+| `legalTargets(board, from)` | `legalTargets(board)(from)` |
+| `legalTargets(board, history, from)` | `legalTargets(board, history)(from)` |
+| `isLegal(board, from, to)` | `isLegal(board)(from, to)` |
+| `isLegal(board, history, from, to)` | `isLegal(board, history)(from, to)` |
+| `isCastle(board, from, to)` | `isCastle(board)(from, to)` |
+| `castlingTargets(board, history, color)` | `castlingTargets(board, history)(color)` |
+| `castleSide(from, to)` | unchanged — no board parameter |
+
+Private helpers (`isOwnPiece`, `isEnemyPiece`, `slide`, `pawnTargets`, `knightTargets`, `kingTargets`) are **not** changed.
+
+### `GameRules` (all public methods)
+
+| Before | After |
+|--------|-------|
+| `isInCheck(board, color)` | `isInCheck(board)(color)` |
+| `legalMoves(board, history, color)` | `legalMoves(board, history)(color)` |
+| `gameStatus(board, history, color)` | `gameStatus(board, history)(color)` |
+
+---
+
+## Section 2: Call Sites Updated
+
+| File | Methods affected |
+|------|-----------------|
+| `modules/core/src/main/scala/de/nowchess/chess/logic/MoveValidator.scala` | Internal calls to `legalTargets`, `castlingTargets`, `isCastle` |
+| `modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala` | `MoveValidator.legalTargets`, `MoveValidator.isCastle`, `isInCheck` |
+| `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala` | `MoveValidator.isLegal`, `MoveValidator.isCastle`, `GameRules.gameStatus` |
+| `modules/core/src/test/scala/de/nowchess/chess/logic/MoveValidatorTest.scala` | `legalTargets`, `isLegal`, `castlingTargets` |
+| `modules/core/src/test/scala/de/nowchess/chess/logic/GameRulesTest.scala` | `isInCheck`, `legalMoves`, `gameStatus` |
+
+`EnPassantCalculator`, `CastlingRightsCalculator`, and their tests are **not** touched.
+
+---
+
+## Section 3: Concrete Style Gain
+
+In `GameRules.isInCheck` and `legalMoves`, `board` is passed to `legalTargets` on every loop iteration today. After currying, it is factored out as a single partial application:
+
+```scala
+// Before
+board.pieces.exists { case (sq, piece) =>
+  piece.color != color &&
+  MoveValidator.legalTargets(board, sq).contains(kingSq)
+}
+
+// After
+val targets = MoveValidator.legalTargets(board)
+board.pieces.exists { case (sq, piece) =>
+  piece.color != color &&
+  targets(sq).contains(kingSq)
+}
+```
+
+---
+
+## Section 4: Testing
+
+Pure refactoring — no new tests. All existing tests must pass after call sites are updated. The test suite is the regression guard.