fix: use mid-point to determine arc direction instead of clockwise flag

The clockwise flag from NX extractor can be inverted depending on face
normal orientation. The sampled mid-point is always reliable. Now
_arc_angles checks which direction (CW vs CCW) passes through the
mid-point and uses that.

tools/adaptive-isogrid/src/shared/arc_utils.py

@@ -16,6 +16,39 @@ def _arc_angles(seg: Dict[str, Any]):
     ex, ey = seg["end"]
     a0 = math.atan2(sy - cy, sx - cx)
     a1 = math.atan2(ey - cy, ex - cx)
+
+    # If a midpoint is available, use it to determine the correct arc direction
+    # (more reliable than the clockwise flag, which can be inverted by face normal orientation)
+    mid = seg.get("mid")
+    if mid is not None:
+        mx, my = float(mid[0]), float(mid[1])
+        a_mid = math.atan2(my - cy, mx - cx)
+
+        # Try CCW direction (a1 > a0)
+        a1_ccw = a1
+        if a1_ccw < a0:
+            a1_ccw += 2.0 * math.pi
+        # Check if mid angle falls within CCW sweep
+        a_mid_ccw = a_mid
+        if a_mid_ccw < a0:
+            a_mid_ccw += 2.0 * math.pi
+        ccw_ok = (a0 <= a_mid_ccw <= a1_ccw)
+
+        # Try CW direction (a1 < a0)
+        a1_cw = a1
+        if a1_cw > a0:
+            a1_cw -= 2.0 * math.pi
+        a_mid_cw = a_mid
+        if a_mid_cw > a0:
+            a_mid_cw -= 2.0 * math.pi
+        cw_ok = (a1_cw <= a_mid_cw <= a0)
+
+        if ccw_ok and not cw_ok:
+            return a0, a1_ccw
+        elif cw_ok and not ccw_ok:
+            return a0, a1_cw
+        # If both or neither match, fall through to clockwise flag
+
     cw = bool(seg.get("clockwise", False))
     if cw:
         if a1 > a0: