🛠️ Fixer: Implement reserved physics features (v0.7.18)

[google-labs-jules]

This PR implements two previously reserved physics features in the LMUFFB engine:

1. Understeer modulation of the Seat of Pants (SoP) effect: Logic added to calculate_sop_lateral to reduce SoP lateral force as front grip is lost. This prevents strong SoP forces from masking understeer, providing a clearer "light wheel" sensation during front-end slides.
2. Configurable Road Fallback scaling: Replaced the hardcoded 0.05 multiplier for vertical-G based road noise (used when suspension telemetry is blocked) with a user-adjustable m_road_fallback_scale.

New GUI controls have been added to src/GuiLayer.cpp:

• "Apply to SoP" toggle under Front Axle (Understeer).
• "Fallback Sensitivity" slider under Tactile Textures -> Road Details (Advanced).

The changes are verified by two new unit tests in tests/test_ffb_reserved_features.cpp, and all 689 tests (including 687 existing regressions) pass successfully. Version has been bumped to 0.7.18.

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PR created automatically by Jules for task 3780225476939567864 started by @coasting-nc

[google-labs-jules]

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