fix(ci): address PR #31 review — harden cross-encoder fixture pipeline

- Fail on encode errors instead of silently writing empty .msgpack files
- Fail test on missing/empty fixture dir instead of silent skip
- Add all missing action variants (MoveSouth, MoveEast, MoveWest,
  Unpause, ToggleStanceDown, WalkAway) to GDScript fixture generator
- Add GDScript fixture staleness check to make pre-pr
- Validate repo root detection before writing outside client/
- Add file.flush() before close in headless mode
- Document fixture failure recovery in DEVOPS.md

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

Makefile

@@ -158,13 +158,13 @@ pre-pr-validate: validate-content check-fact-ids
 	@echo "--- Content validation: PASS ---"
 
 pre-pr-fixtures:
-	@echo "Checking fixture staleness..."
+	@echo "Checking Rust->GDScript fixture staleness..."
 	@cd server && cargo test --test gen_fixtures -- --ignored
 	@if git diff --quiet client/tests/fixtures/; then \
-		echo "--- Fixtures: UP TO DATE ---"; \
+		echo "--- Rust fixtures: UP TO DATE ---"; \
 	else \
 		echo ""; \
-		echo "--- FIXTURES STALE ---"; \
+		echo "--- RUST FIXTURES STALE ---"; \
 		echo "  Protocol changed but fixtures not regenerated."; \
 		echo "  Stale fixtures make all client tests FALSE POSITIVES."; \
 		echo ""; \
@@ -174,6 +174,22 @@ pre-pr-fixtures:
 		echo "  Fix: commit the updated fixtures with your protocol change."; \
 		exit 1; \
 	fi
+	@echo "Checking GDScript->Rust fixture staleness..."
+	@$(MAKE) fixtures-client
+	@if git diff --quiet server/tests/fixtures/gdscript/; then \
+		echo "--- GDScript fixtures: UP TO DATE ---"; \
+	else \
+		echo ""; \
+		echo "--- GDSCRIPT FIXTURES STALE ---"; \
+		echo "  Protocol changed but GDScript fixtures not regenerated."; \
+		echo "  Stale fixtures make cross-encoder tests FALSE POSITIVES."; \
+		echo ""; \
+		echo "  Changed files:"; \
+		git diff --stat server/tests/fixtures/gdscript/; \
+		echo ""; \
+		echo "  Fix: commit the updated fixtures with your protocol change."; \
+		exit 1; \
+	fi
 
 # Branch-specific variants (faster, scope-appropriate)