Release v4.6.1

[gdifiore]

Summary

• Fix roll rest detection so low-speed balls that are still accelerating downhill do not prematurely end the roll phase.
• Fix aerial phase timing so custom integrators receive the documented start-of-step currentTime, matching bounce phase behavior.
• Add regression coverage for downhill roll continuation and custom integrator timing.

Verification

• cmake --build build --target libgolf_tests && ./build/libgolf_tests
• 133 tests passed.