LinuxCNC S-Curve Accelerations

The proposal on the table is a new modal G64.1 P Q
jerk-aware continuous path mode that behaves like G64 P Q but adds automatic full stop fallback when the jerk-constrained cornering velocity rounds to zero. Off by default to preserve existing behavior, opt-in for integrators who need provably clean command streams or are running drives without internal interpolation. That seems like the right balance between being true to the physics and being useful across the wide range of hardware LinuxCNC runs on.