Expose cargo and passenger UI actions

[sviyh]

Summary

• refactor cargo and passenger action boundaries for Multiplayer compatibility
• expose stable named actions for the vehicle cargo dialog
• expose passenger transfers with explicit pawn, hovered pawn, and target-holder inputs
• preserve the existing UI behavior while allowing Multiplayer compatibility to bind public methods instead of compiler-generated lambdas and UI-static drag state

Root cause

Cargo and passenger mutations were embedded in UI delegates or depended on static drag fields. External integrations therefore had to identify compiler-generated callbacks or reconstruct state that the Vehicle Framework already owns.

Approach

• route the load-cargo gizmo through VehiclePawn.OpenLoadCargoDialog
• extract Accept, Reset, Cancel, PackInstantly, and SetToSendEverything from Dialog_LoadCargo
• route passenger drag completion through VehicleTabHelper_Passenger.TransferPawn and pass the pawn/holders explicitly through the seat-change bookkeeping

The original UI paths still call these methods; the change only provides stable action boundaries.

Test plan

• Release build succeeds with no warnings or errors
• Rebuild Vanilla Vehicles Expanded against this exact Vehicle Framework build
• Manual single-player: open the cargo dialog; exercise Reset, Cancel, Accept, cancel-loading, select-everything, and instant packing
• Manual single-player: transfer passengers between compatible handlers, swap occupied handlers, and verify rejection paths leave assignments unchanged
• Manual single-player: save/reload after cargo and passenger changes and verify holders/counts remain correct