perf(core): reduce inter-service HTTP calls from 11 to 4 per move

- Add `postMoveStatus` batch method to `RuleSet` trait (default impl composes
  individual calls; `RuleSetRestAdapter` overrides with single HTTP round-trip)
- Collapse 5 sequential rule checks in `GameEngine.executeMove` into one
  `postMoveStatus` call
- Add `POST /api/rules/post-move-status` endpoint to rule-service
- Add `exportCombined` to `IoServiceClient` and `POST /io/export/combined`
  endpoint to io-service, replacing two separate FEN/PGN HTTP calls
- Fix `statusOf` to pattern-match on `WinReason` from `ctx.result` instead
  of making a redundant `isCheckmate` HTTP call
- Remove duplicate `legalMoves` pre-validation in `GameResource.makeMove`;
  engine already validates and fires `InvalidMoveEvent`
- Add `scalafix:off` guards for pre-existing `var`/`return` usage in
  `DefaultRules` hot-path code
- Apply spotless formatting to previously unformatted files

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

modules/api/src/main/scala/de/nowchess/api/rules/PostMoveStatus.scala

@@ -0,0 +1,9 @@
+package de.nowchess.api.rules
+
+final case class PostMoveStatus(
+    isCheckmate: Boolean,
+    isStalemate: Boolean,
+    isInsufficientMaterial: Boolean,
+    isCheck: Boolean,
+    isThreefoldRepetition: Boolean,
+)

modules/api/src/main/scala/de/nowchess/api/rules/RuleSet.scala

@@ -39,3 +39,15 @@ trait RuleSet:
     * promotion. Updates castling rights, en passant square, half-move clock, turn, and move history.
     */
   def applyMove(context: GameContext)(move: Move): GameContext
+
+  /** Batch status check after a move is applied. Replaces individual isCheckmate/isStalemate/isInsufficientMaterial/
+    * isCheck/isThreefoldRepetition calls with a single round-trip. Override for remote implementations.
+    */
+  def postMoveStatus(context: GameContext): PostMoveStatus =
+    PostMoveStatus(
+      isCheckmate = isCheckmate(context),
+      isStalemate = isStalemate(context),
+      isInsufficientMaterial = isInsufficientMaterial(context),
+      isCheck = isCheck(context),
+      isThreefoldRepetition = isThreefoldRepetition(context),
+    )