Keep twist poles invariant under angular speed scaling (#249)

`Twist2.pole` and `Twist3.pole` divide the cross product by the angular
magnitude. The closest point on the screw axis requires its square: the
cross product already contains one factor of angular speed.
Consequently, scaling a twist currently moves its reported axis.

```python
from spatialmath import Twist2, Twist3

print(Twist2([1, -2, 0.5]).pole)        # expected [4, 2]; currently [2, 1]
print(Twist3([1, -2, 0, 0, 0, 0.5]).pole)  # expected [4, 2, 0]
```

Use `w**2` in 2D and `dot(w, w)` in 3D. The zero-angular case keeps its
existing undefined behavior.

The regression checks cover signed speeds, off-origin axes and nonzero
screw pitch. They verify that finite motion leaves an axis point fixed
except for translation along the axis. Twelve of the 15 cases fail
before the fix. On a camera-reconstruction fixture, 680 incorrect axis
estimates become zero across 768 scaled 2D/3D increments; independent
least-squares axis references agree within `1.5e-14`. Ordinary transform
outputs remain identical. Full suite: 362 passed, 3 existing skips;
project pre-commit hooks pass. Linux/Python 3.12.

Author: antonsoo

spatialmath/twist.py

@@ -986,7 +986,7 @@ def pole(self):
             >>> S = Twist3(T)
             >>> S.pole
         """
-        return np.cross(self.w, self.v) / self.theta
+        return np.cross(self.w, self.v) / np.dot(self.w, self.w)
 
     def SE3(self, theta=1, unit="rad"):
         """
@@ -1502,7 +1502,7 @@ def pole(self):
             >>> S.pole
 
         """
-        p = np.cross(np.r_[0, 0, self.w], np.r_[self.v, 0]) / self.theta
+        p = np.cross(np.r_[0, 0, self.w], np.r_[self.v, 0]) / self.w**2
         return p[:2]
 
     # -------------------------  methods -------------------------------#

tests/test_twist_pole.py

@@ -0,0 +1,35 @@
+import numpy as np
+import pytest
+from scipy.linalg import expm
+
+from spatialmath import Twist2, Twist3
+
+
+@pytest.mark.parametrize("speed", [-2.0, -0.3, 0.3, 1.0, 2.0])
+def test_twist2_pole_is_rotation_center(speed):
+    """A change in angular speed must not move the center of rotation."""
+    center = np.array([3.0, -2.0])
+    twist = Twist2(speed * np.r_[center[1], -center[0], 1])
+    np.testing.assert_allclose(twist.pole, center)
+    transform = expm(twist.skewa() * 0.7)
+    np.testing.assert_allclose(
+        transform @ np.r_[twist.pole, 1], np.r_[center, 1], atol=1e-14
+    )
+
+
+@pytest.mark.parametrize("speed", [-2.0, -0.3, 0.3, 1.0, 2.0])
+@pytest.mark.parametrize("pitch", [0, 0.4])
+def test_twist3_pole_is_on_screw_axis(speed, pitch):
+    """The closest axis point is fixed except for translation along the axis."""
+    axis = np.array([1.0, 2.0, -2.0]) / 3
+    center = np.array([3.0, -2.0, 1.0])
+    center -= (center @ axis) * axis
+    angular = speed * axis
+    linear = -np.cross(angular, center) + pitch * angular
+    twist = Twist3(linear, angular)
+    np.testing.assert_allclose(twist.pole, center)
+    transform = expm(twist.skewa() * 0.7)
+    expected = center + pitch * angular * 0.7
+    np.testing.assert_allclose(
+        (transform @ np.r_[twist.pole, 1])[:3], expected, atol=1e-14
+    )