Preserve proper motion for nearly radial trajectories

[sylvesterkaczmarek]

Summary

Compute transverse speed with a cross product instead of subtracting nearly equal squared speeds. For a 7500 m/s radial component and 1e-6 m/s transverse component at 7e6 m, proper_motion() previously returned zero instead of 2.946642857142857e-8 in the existing SI output scale. Observer offsets and unit conversion are unchanged.

Repository Policy Checklist

• All review gates in AGENTS.md are complete. Machine-generated code and tests; executed checks and remaining gates are recorded below.
• Changes preserve the existing file layout.
• Package-code changes include tests under tests/.
• No new workflow or demo is needed; this corrects existing numerical behavior.
• No generated outputs, data, images, or binary artifacts are committed.
• No persistent data is added.

Validation

• Full pytest suite is green; see results below.
• Full demo gallery run (not run).
• Repository policy check inspects both changed paths.
• No existing test, tolerance, workflow, or policy check was weakened.
• Every numerical result below came from an executed command.

Python 3.12, macOS arm64; published SSAPy 1.1.10 and SSAPy-Data 0.1.5. Ruff, compilation, diff check, warning-as-error Sphinx build, wheel/sdist builds, and twine check passed.

New regression cases: 6 failed and 4 passed before the fix; all 10 pass afterward. The focused mechanics suite passes 14 tests, including 100 independent tangent-basis comparisons. Full suite: 981 passed, 20 skipped, 4 failed; 43 subtests passed.

The same four full-suite failures reproduce on unchanged upstream 262b43c: orbital-maneuver demo, sibling data-directory resolution, orbit-STM finite differences, and multi-segment 6-DoF targeting. They are outside this change. The existing failures remain unresolved; upstream CI must still be checked.