Truly Best Friends Forever

Optimize pet data persistence to eliminate server-side tick stutter.

Fix the periodic stutter caused by flushPendingPetSaves in the server-side onServerTick:
Every tick, it performs full NBT serialization for all loaded pets + reading old files + deep
equals comparison + atomic writing of temporary files, even when the data has not changed.

• Added PetSnapshotFingerprint: records the content fingerprint of the most recent persisted
  snapshot by pet UUID (FNV-style structural hash). Before writing, compare the fingerprint first;
  if the content has not changed, skip reading the old file and disk writing entirely.

• Added a PetStateSignature pre-check to savePetData: if the coordinates (1/16 block
  quantization), orientation, health, name, sitting state, and AI state have all not changed, and
  there is already a pending persistence task, skip the full entity NBT collection in
  petEntitySnapshot.

• Added an isPetSyncCandidate pre-filter to collectLocalSyncCandidates: only collect candidates
  for already-tracked pets or entities whose owner can be resolved, instead of enqueueing all
  irrelevant entities such as items and projectiles within a 5x5 chunk area.

• Added a 10-second TTL negative cache to hasPetFileInOtherOwnerDir to avoid traversing all
  player directories every tick.

• Unified cache cleanup: added forgetPetCaches, which clears petDeathTimes,
  petStateSignatures, noForeignOwnerFileUntil, and the fingerprint table when pet data is
  cleared/deleted; clears all fingerprint caches when the server stops.

The fingerprint cache exists only in memory and is cleared when the server stops. Scenarios where
NBT files are directly modified offline will still take effect normally on the next startup,
preserving the original behavior unchanged.