fix(rs274ngc): MAX_UNWIND_TURNS rebase aligns angular position via shortest path

The original rebase pinned the motor at AA_current when triggered, which
gave correct results only when AA_current happened to coincide with the
target's mod-360 angular position (e.g. an exactly N-turn accumulation).
For any off-mod-360 starting state, the motor would not move and the
physical angular position would no longer match the user-frame target.

Updated rebase computes the shortest-path delta into (-180, 180] and
moves the motor by that sub-turn delta while absorbing the full-turn
remainder in the offset. After rebase, machine angular position == target
mod 360 and user_pos == target.

Trace from AA=28850 (80 turns + 50 deg) with G0 A0:
  delta = -28850, |80.14 turns| > 10, rebase fires
  delta_short = -50 (shortest path to align mod 360)
  target_machine = 28800, motor delta = -50
  new_offset = 28800, user_pos = 0
  Physical angular position = 0 deg, matches programmed A0.

Author: grandixximo

src/emc/rs274ngc/interp_convert.cc

@@ -5345,8 +5345,11 @@ int Interp::convert_straight(int move,   //!< either G_0 or G_1
   settings->motion_mode = move;
 
   // Auto-rotary-rebase: on G0 with a rotary word, if the user-frame delta
-  // exceeds MAX_UNWIND_TURNS, shift the modulo offset so the motor stays
-  // put while the user-frame position jumps to the programmed target.
+  // exceeds MAX_UNWIND_TURNS, shift the modulo offset so the motor takes the
+  // shortest path to the target's angular position (within +/- 180 deg)
+  // while the user-frame position jumps to the programmed target. The whole
+  // turns of unwind are absorbed by the offset; only the sub-turn part is
+  // physical motion.
   // Only applies in absolute mode (G90); G53 explicitly bypasses; incremental
   // (G91) does not need it.
   if (move == G_0 &&
@@ -5358,7 +5361,11 @@ int Interp::convert_straight(int move,   //!< either G_0 or G_1
           double user_pos = cur - *off;
           double delta = bnum - user_pos;
           if (fabs(delta) / 360.0 > max_turns) {
-              *off = cur - bnum;
+              double mod = fmod(delta + 180.0, 360.0);
+              if (mod < 0.0) mod += 360.0;
+              double delta_short = mod - 180.0;
+              double target_machine = cur + delta_short;
+              *off = target_machine - bnum;
           }
       };
       rebase(settings->AA_current, &settings->AA_modulo_offset,