feat(ci): add make fixtures-client target for GDScript->Rust cross-encoder validation (#475)

Closes the bidirectional protocol compatibility loop (D-030 Layer 1):
- GDScript fixture generator (20 fixtures: inputs, boundary ticks, batch)
- Rust decoder test verifying all GDScript-encoded fixtures deserialize
- Makefile target with generation + verification in one step

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

Makefile

@@ -4,7 +4,8 @@ GODOT := $(shell command -v godot4 2>/dev/null || command -v godot 2>/dev/null)
        decisions-sync decisions-coverage decisions-active decisions-orphan \
        db-backup db-install validate-content content-ron check-fact-ids setup-hooks \
        pre-pr pre-pr-lint pre-pr-build pre-pr-test pre-pr-validate pre-pr-fixtures \
-       pre-pr-server pre-pr-client pre-pr-content
+       pre-pr-server pre-pr-client pre-pr-content \
+       fixtures-client
 
 # --- Configuration ---
 
@@ -37,6 +38,7 @@ help:
 	@echo "  make validate-content   Validate content YAML against schemas"
 	@echo "  make check-fact-ids    Check fact_id references against knowledge catalogs"
 	@echo "  make content-ron       Convert content YAML to RON (build-time)"
+	@echo "  make fixtures-client    Generate GDScript->Rust cross-encoder fixtures (#475)"
 	@echo ""
 	@echo "  make pre-pr             Run all pre-PR checks (lint, build, test, validate, fixtures)"
 	@echo "  make pre-pr-server      Server-scoped pre-PR (lint, build, test, fixtures)"
@@ -111,6 +113,14 @@ test-server:
 fixtures:
 	cd server && cargo test --test gen_fixtures -- --ignored
 
+fixtures-client:
+	@test -n "$(GODOT)" || { echo "Godot not found. Run 'make setup' first."; exit 1; }
+	@echo "Generating GDScript fixtures for Rust decoder..."
+	$(GODOT) --headless --path client -s res://tests/gen_client_fixtures.gd
+	@echo "Verifying Rust can decode GDScript fixtures..."
+	cd server && cargo test --test serialization gdscript_generated_fixtures
+	@echo "--- Fixtures-client: PASS ---"
+
 test-client:
 	@test -n "$(GODOT)" || { echo "Godot not found. Run 'make setup' first."; exit 1; }
 	$(GODOT) --headless --path client -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd --ignoreHeadlessMode -a res://tests/