fix(animation): variant randomness no longer permanently cached + fix FullHead false-positive

Two fixes from the animation audit:

1. Variant cache key now includes the resolved animation name (e.g., Struggle.2).
   Previously, the cache key only used context+parts, so the first random variant
   pick was reused forever. Now each variant gets its own cache entry, and a fresh
   random pick happens each time the context changes.

2. FullHead check changed from contains("Head") to startsWith("gltf_FullHead")
   to prevent false positives on names like FullOverhead or FullAhead.

src/main/java/com/tiedup/remake/client/gltf/GltfPoseConverter.java

@@ -587,7 +587,10 @@ public final class GltfPoseConverter {
         // Head is protected by default — only enabled as a free bone when the animation
         // name contains "Head" (e.g., FullHeadStruggle, FullHeadIdle).
         // This lets artists opt-in per animation without affecting the item's regions.
-        boolean allowFreeHead = isFullBodyAnimation && animName.contains("Head");
+        // FullHead prefix (e.g., FullHeadStruggle) opts into head as a free bone.
+        // Use startsWith to avoid false positives (e.g., FullOverhead, FullAhead).
+        boolean allowFreeHead = isFullBodyAnimation &&
+            animName.startsWith("gltf_FullHead");
 
         String[] allParts = {
             "head",