feat: per-entity targeting — spoof a specific entity, not all of a serializer

Unified registry keyed by (ser, field, entityIndex): -1 = all entities (prior
behavior), >=0 = one entity. ValueCopyHook two-step lookup: entity-specific
(_currentEntityIndex) wins, global fallback. API: HookEntityInt / SetEntitySpoof
/ UnhookEntity. SUBST diag logs ent/target for live validation. Example:
sp_example_fakehp_entity <idx> <value>. Thread-safe (ConcurrentDictionary +
ThreadStatic). Build 0/0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>