Fix Line3 distance for parallel and non-unit directions

[Doribelove]

Line3.distance() raises NameError for parallel lines and can return incorrect distances for skew or intersecting lines. For example:

from spatialmath import Line3

a = Line3.PointDir([0, 0, 0], [1, 0, 0])
b = Line3.PointDir([0, 0, 2], [0.6, 0.8, 0])
print(a.distance(b))  # before: 2.5; expected: 2.0

The old expression divides by the squared cross-product norm and uses a reciprocal product that mixes unit directions with unnormalized moments. Scaling directions can therefore change the result; a translated intersecting pair can also return a nonzero distance.

Compute the displacement between principal points and project it onto the common normal of the unit directions. For parallel directions, use the perpendicular component of that displacement and return a scalar. Apply the existing tol parameter to the parallel-direction test.

Validation on base f6a572c6e75e2a7e3c06e632bc34153941e00404:

• Added regressions with the original implementation: 21 failed, 18 passed in tests/test_geom3d.py; with the fix: 39 passed.
• Full tests suite: baseline 346 passed, 3 skipped; fixed 367 passed, 3 skipped, on Python 3.10/NumPy 2.2.6 and Python 3.12/NumPy 2.5.3. Existing plotting/deprecation warnings remain.
• Separate seeded validation: 4,800 scalar comparisons per environment against closest points computed by numpy.linalg.lstsq, including parallel/intersecting/skew lines, sign/scale changes, rigid transformations and operand reversal.
• Pinned pre-commit hooks on both changed files, git diff --check, wheel and sdist builds passed.

AI assistance: the implementation, tests and this description were prepared with Codex; the validation commands were executed locally. No full multi-platform CI result is claimed.