This feature adds a "Call Unloader" button to manually-driven combines (i.e. combines that the player is driving themselves, not Courseplay-controlled). When activated, a nearby Courseplay-managed grain cart automatically:

1. Approaches the combine via pathfinding
2. Positions itself under the combine's unloading pipe
3. Follows the combine while it harvests (including through gentle curves and S-bends)
4. Leaves when the player closes the pipe (2-second debounce prevents false exits)
5. Automatically re-approaches if it loses position

The design goal is that the player only touches the button once — the grain cart handles everything until the pipe is closed.

Note: This PR replaces #1243, which was closed because it included unrelated personal preferences (US unit display). This branch contains only the manual unloader feature.

Files Changed

Key Design Decisions

CpManualCombineProxy implements the full AIDriveStrategyCombineCourse interface so AIDriveStrategyUnloadCombine can interact with a manual combine using identical method calls — no nil checks or special-casing scattered through the unloader code.

isManualProxy() → true — marker method used to gate manual-only behavior in AIDriveStrategyUnloadCombine without re-querying the vehicle.

getFillLevelPercentage() → 1 — always reports full so fill level never causes the cart to leave. The only valid exit is isUnloadFinished() (pipe closed for 2+ seconds).

driveBesideCombine() direct goal point — for manual combines, a live goal point is computed every frame from the combine's AI direction node at the pipe's lateral offset, normalLookAheadDistance meters ahead. As the combine turns, the node rotates and the cart follows naturally through curves and gentle S-bends. The placeholder course (built from combine's current heading) exists only to satisfy PPC initialisation and is never consulted for steering.

PPC soft-recovery hook — onOffTrackShutdown() in AIDriveStrategyUnloadCombine transitions the cart to IDLE instead of calling stopCurrentAIJob. The proxy's callUnloaderWhenNeeded() re-summons within ~2.5 s. The user never has to manually restart the grain cart.

Off-track grace period — offTrackGracePeriodMs = 10000 (class default, unchanged). During active unloading, disableStopWhenOffTrack(5000) is called each tick, fully suppressing off-track detection while the cart is under the pipe.

Forage harvesters excluded — button and keybind are hidden when pipeSpec.numAutoAimingStates > 0. Their auto-aim spout geometry requires a separate implementation.

Backward Compatibility

All changes are backward-compatible with CP-driven combines:

• Manual-only steering code is gated by combineStrategy:isManualProxy()
• Shared fixes (proximity terrain ignore, PPC soft-recovery hook) benefit CP-driven unloaders as well
• The callUnloaderPercent slider change (min 60% → 20%) applies to all combines

Known Limitations

• Sharp turns (>~60°) cause the cart to lose tracking; soft recovery re-approaches within a few seconds
• Forage harvesters are explicitly unsupported (button hidden)
• Requires the grain cart to be running a Courseplay Combine Unloader job