Expose NPC vehicle lifecycle events

[diffuin]

Requested by @ifBars in #243.

Diffuin response

Confidence: Medium

Implemented PR-ready NPC vehicle entry and exit events with cached dual-runtime delegate bridges and scene-cleanup detachment. Pull request creation is not available in this environment; the patch fully resolves issue #243 when submitted.

Recommended next steps

1. Added documented Action events without changing existing API members.
2. Bridges attach while either managed event has handlers and detach using the same cached delegate instances.
3. The events add no persistence or S1API networking; they follow native callback delivery.

Evidence and validation

Evidence inspected

• Regular and beta Mono NPC declarations expose public Action onEnterVehicle and onExitVehicle fields.
• NPC.CurrentVehicle already uses the direct LandVehicle conversion path.
• NPCMessaging and NPCRelationship provide the cached IL2CPP delegate conversion and removal pattern used by this patch.

Validation performed

• Read repository, modding, implementation, and writing guidance.
• Verified native callback declarations in regular and beta stripped source.
• Ran git diff --check and reviewed the final two-file diff.
• Confirmed no .NET SDK, MSBuild, or C# compiler is available.

Runtime validation remaining

• Run restore, build, and test for MonoMelon and Il2CppMelon with local game references.
• In both runtimes, verify callback timing, unsubscription, NPC destruction, and scene transitions.
• Verify host and remote-client callback delivery. No multiplayer, save/load, Unity runtime, or end-to-end validation ran.

Diffuin run details

• Provider: codex
• Model: gpt-5.6-terra
• Reasoning: high (Luna advisor: balanced (high); baseline bounded implementation)
• Elapsed: 487s
• Codex thread: 019ff8ea-43ee-7a52-b3a8-d57cfe2d4298

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AI notice: Generated with AI assistance and not guaranteed accurate. Verify findings and plans against the current source and runtime.

Closes #243

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Diffuin job: 29624926-e1bd-4f77-9837-2d947aa9a1d2
Commit: c3d31e4652bd208e2c8aa81c4345cf1adf986c27