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.

Initial 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.

The branch is now rebased onto upstream 2de7801 (current head 9600087). On this updated tree, the complete tests suite passes with 368 passed, 3 skipped on Python 3.10.12, and the four pre-commit hooks on both changed files and git diff --check pass. The newer upstream commit adds a separate spatial-vector regression, accounting for one more passing test than the initial run. The GitHub Actions CI run still requires maintainer approval.

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.

[Doribelove]

I rebased this PR onto current master (2de7801), so it is no longer behind the target branch. On the new head (9600087), the complete local test suite passes with 368 passed and 3 skipped; the changed-file pre-commit hooks and git diff --check also pass. The new GitHub Actions CI run is marked action_required and needs approval from a repository maintainer before it can execute. Could someone approve that run and review the fix when convenient? Thank you.