feat(rs274ngc): MAX_UNWIND_TURNS auto rotary G0 rebase

[grandixximo]

Draft for testing and feedback. Targets the rotational-cutting use case from #3902 where G1 must honor literal multi-turn absolute targets (helical winding, spiral carving) but a terminal G0 should not physically unwind.

Mechanism

New INI flag [AXIS_<L>] MAX_UNWIND_TURNS = N. On a G0 absolute move with the rotary word, if the user-frame delta exceeds N full turns, the interpreter shifts a per-axis hidden user-frame offset so the motor stays put while the user-frame position jumps to the programmed target. Subsequent absolute moves use the rebased frame.

The motion-side traj.position remains accumulated. Only an interp-only offset moves. Stepgens, encoders, and PID see no discontinuity, sidestepping the motor_offset injection problem @andypugh raised in #3902. G53 explicitly bypasses (literal machine coords). Incremental (G91) is unaffected.

Trace

[AXIS_A] MAX_UNWIND_TURNS = 10

state: AA=0, offset=0, user=0
G0 A0 X0 Y5 Z1     ; no trigger (|delta|=0)
G1 Y0
G1 X20 A28800      ; literal target = 28800, +28800 (80 turn cut)
                   ; AA=28800, offset=0, user=28800
G0 X0 A0           ; |delta_turns|=80 > 10, rebase: offset=28800
                   ; machine_target=28800, motor delta=0 (no unwind)
                   ; AA=28800, offset=28800, user=0

#5423/#5424/#5425 and _a/_b/_c named params report user-frame.

Trade-offs / caveats

• Absolute machine-frame position drifts by one rebase per run. MAX_LIMIT eventually reached on very long runs. Trade is necessary because in-program joint rebase is not safely doable (per @andypugh's analysis).
• Trivkins 1:1 axis-to-joint only. Coupled kinematics (5-axis TCP, gantry rotary) unsupported (not validated).
• Run-from-line not handled (skipped blocks would not trigger rebases).
• Offset persists across M2/M30 (clean display for back-to-back runs). The first G0 of the next program self-heals after re-home/estop/power-cycle because the unwind delta exceeds the threshold.
• Mutex with WRAPPED_ROTARY, startup warning if both set.

Mapping to other controls

Closest precedent is Hurco M31 (explicit, programmer-placed). This implementation auto-triggers on G0 above a threshold; to my knowledge no commercial control automates this but the structural reasons (mostly TCP coupling) are addressable via the trivkins gate.

Status

Draft, seeking feedback and test results from @djdelorie's helical-carving setup before promoting. Build clean.