Expose managed land-vehicle seat occupancy

[diffuin]

Requested by @ifBars in #294.

Diffuin response

Confidence: Medium

Implemented the approved seat metadata and occupancy API. The additive change exposes cached read-only seat wrappers, managed occupant change events, and native assignment observation without exposing game seat or player types. Existing public members and persistent or network identities are unchanged.

Recommended next steps

1. LandVehicle.Seats preserves native order and caches VehicleSeatInfo instances.
2. Occupants resolve only through Player.All by native identity, returning null when no managed wrapper exists.
3. The Harmony postfix observes native mutations and suppresses only same-native-player transitions.

Evidence and validation

Evidence inspected

• Regular and beta stripped sources both expose ordered vehicle seats, driver metadata, player occupants, and the private SetSeatOccupant path.
• Serialized Main scene data contains explicit isDriverSeat values.
• The existing vehicle registry removes cached wrappers on native vehicle destruction; the new seat state is owned by those wrappers.

Validation performed

• Read the required modding, implementation, and writing guidance.
• Read issue [FEATURE] Expose vehicle seat metadata and occupancy #294 and its approved plan through read-only GitHub evidence.
• Inspected regular and beta stripped source declarations plus serialized seat metadata.
• Ran git diff --check.,dotnet` is unavailable in this environment.

Runtime validation remaining

• Restore, build, and run contract tests for MonoMelon and Il2CppMelon with local game references.
• Validate driver/passenger transitions, disconnects, vehicle destruction, wrapper cleanup, and event timing in both runtimes.
• Confirm host and remote-client observations in a human-run multiplayer session.

Diffuin run details

• Provider: codex
• Model: gpt-5.6-terra
• Reasoning: high (Luna advisor: complexity (high); baseline bounded implementation)
• Elapsed: 610s
• Codex thread: 01a06e6e-27d5-7e83-a71f-d6fdf249c07c

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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 #294

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Diffuin job: 78db5017-c1ac-4f14-a5c5-cb6567254cac
Commit: ed18cf6da551f3780bc205babb60d461015c0a82