feat(creatures): animate creature walk cycles

Add a Gait component synchronized over the network and procedural animation for model nodes. Load animation parameters from assets/model/creature/pig/animation.json and apply leg swing, body bob, and head motion based on gait. Refactor Gait into its own header for reuse.

assets/model/creature/pig/animation.json

@@ -0,0 +1,33 @@
+{
+  "walk": {
+    "speed": 6.0,
+    "amplitude": 25.0
+  },
+  "run": {
+    "speed": 12.0,
+    "amplitude": 40.0
+  },
+  "body_bob": 0.03,
+  "head": {
+    "node": "Head",
+    "amplitude": 4.0
+  },
+  "legs": [
+    {
+      "node": "Foot_FL",
+      "phase": 0.0
+    },
+    {
+      "node": "Foot_FR",
+      "phase": 3.14159
+    },
+    {
+      "node": "Foot_HL",
+      "phase": 3.14159
+    },
+    {
+      "node": "Foot_HR",
+      "phase": 0.0
+    }
+  ]
+}