CAM: Add threshold for treating large-radius arcs as linear in simulator

Arc segments with extremely large radius compared to chord length can
cause floating-point precision issues in simulation. This change
introduces ARC_LINEARIZATION_THRESHOLD to treat such arcs as linear,
improving numerical stability in the simulator.

The following g-code was causing erractic behavior in the simulator due
to large-radius arcs:
```
G2 I88775.835760 J-1936991.545892 K0.000000 X102.063107 Y100.102815 Z12.700001
```

src/Mod/CAM/PathSimulator/AppGL/MillPathSegment.cpp:
- Added ARC_LINEARIZATION_THRESHOLD constant
- Updated arc motion detection to treat large-radius arcs as linear
- Improved handling of arc vs. linear segment classification

src/Mod/CAM/PathSimulator/AppGL/MillPathSegment.cpp

@@ -39,6 +39,11 @@ constexpr auto pi = std::numbers::pi_v<float>;
 #define PY 1
 #define PZ 2
 
+// Maximum ratio of radius to chord length for treating arc as curved
+// Ratios above this indicate the arc is essentially a straight line
+// and should be treated as linear to avoid numerical precision issues
+constexpr float ARC_LINEARIZATION_THRESHOLD = 100000.0f;
+
 
 namespace MillSim
 {
@@ -71,9 +76,21 @@ MillPathSegment::MillPathSegment(EndMill* _endmill, MillMotion* from, MillMotion
     mXYAngle = atan2f(mDiff[PY], mDiff[PX]);
     endmill = _endmill;
     mStartAngRad = mStepAngRad = 0;
-    if (IsArcMotion(to)) {
-        mMotionType = MTCurved;
+
+    // Check if this is an arc motion and whether it should be treated as curved
+    bool isArc = IsArcMotion(to);
+    bool treatAsCurved = false;
+
+    if (isArc) {
         mRadius = sqrtf(to->j * to->j + to->i * to->i);
+
+        // Check if arc is essentially a straight line by comparing radius to chord length
+        // When radius >> chord length, floating-point precision issues occur in angle calculations
+        treatAsCurved = (mRadius <= mXYDistance * ARC_LINEARIZATION_THRESHOLD);
+    }
+
+    if (treatAsCurved) {
+        mMotionType = MTCurved;
         mSmallRad = mRadius <= endmill->radius;
 
         if (mSmallRad) {