Expose NPC vehicle lifecycle events

[diffuin]

Summary

• Exposes NPC.OnEnterVehicle and NPC.OnExitVehicle with managed LandVehicle arguments.
• Subscribes each mod handler directly to the corresponding native NPC delegate through a retained Mono/IL2CPP bridge.
• Removes the exact native registration on event removal and cleans remaining registrations during NPC destruction or scene reset.
• Lazily initializes registration state so joined-client wrappers created without constructors remain supported.

Validation

• MonoMelon full suite: 578 passed.
• Il2CppMelon full suite: 567 passed.
• Mono gameplay smoke: passed in Main against head c3e9771.
• IL2CPP gameplay smoke: passed in Main against head c3e9771.
• Runtime probe verified native enter/exit delivery, managed vehicle conversion, duplicate registration, one-at-a-time removal, final unsubscription, and throwing-subscriber isolation.
• git diff --check passed.

Closes #243