feat(engine): live server visual tests and gauntlet snapshot replay

Add live server lifecycle to tests/run-visual (start/stop server per
scenario, parse LISTENING:{port}). Add MessagePack snapshot replay to
visual_capture.gd via Protocol.decode_snapshot() — exercises the full
client pipeline from wire bytes to rendered fog. Three replay scenarios
(hub_spawn, fog_theater, hub_after_movement) plus one live scenario
(fog_live_hub). Add gen_gauntlet_fixtures.rs to produce .msgpack fixtures
from the Gauntlet test world. Add max_diff_pct threshold to visual-diff.
Makefile: add fixtures-gauntlet target, fix build-client double-import,
preserve .godot cache in clean.

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

tooling/visual-diff

@@ -239,18 +239,22 @@ def compare(
 # ---------------------------------------------------------------------------
 
 
-def load_tolerance(config_path: Path | None) -> int:
-    """Read tolerance from config JSON, return DEFAULT_TOLERANCE on failure."""
+def load_config(config_path: Path | None) -> dict:
+    """Read visual test config, return dict with tolerance and max_diff_pct."""
     if config_path is None:
         config_path = DEFAULT_CONFIG
+    defaults = {"tolerance": DEFAULT_TOLERANCE, "max_diff_pct": 0.0}
     if not config_path.exists():
-        return DEFAULT_TOLERANCE
+        return defaults
     try:
         with open(config_path) as f:
             data = json.load(f)
-        return int(data.get("tolerance", DEFAULT_TOLERANCE))
+        return {
+            "tolerance": int(data.get("tolerance", DEFAULT_TOLERANCE)),
+            "max_diff_pct": float(data.get("max_diff_pct", 0.0)),
+        }
     except (json.JSONDecodeError, ValueError, OSError):
-        return DEFAULT_TOLERANCE
+        return defaults
 
 
 # ---------------------------------------------------------------------------
@@ -270,6 +274,12 @@ def main() -> int:
         default=None,
         help="Per-channel pixel tolerance (default: from config or 5)",
     )
+    parser.add_argument(
+        "--max-diff-pct",
+        type=float,
+        default=None,
+        help="Max allowed diff percentage (default: from config or 0.0)",
+    )
     parser.add_argument(
         "--diff-output",
         default=None,
@@ -282,9 +292,11 @@ def main() -> int:
     )
     args = parser.parse_args()
 
-    # Resolve tolerance: CLI > config > fallback
+    # Resolve settings: CLI > config > fallback
     config_path = Path(args.config) if args.config else None
-    tolerance = args.tolerance if args.tolerance is not None else load_tolerance(config_path)
+    cfg = load_config(config_path)
+    tolerance = args.tolerance if args.tolerance is not None else cfg["tolerance"]
+    max_diff_pct = args.max_diff_pct if args.max_diff_pct is not None else cfg["max_diff_pct"]
 
     # Read images
     try:
@@ -322,6 +334,11 @@ def main() -> int:
         return 0
 
     pct = diff_count / total * 100
+
+    if pct <= max_diff_pct:
+        print(f"PASS: {diff_count} of {total} pixels differ ({pct:.1f}%, within {max_diff_pct}% threshold)")
+        return 0
+
     print(f"FAIL: {diff_count} of {total} pixels differ ({pct:.1f}%)")
 
     if args.diff_output and diff_buf: