✨ feat(vectors): add the Point overload of chord_distance

[nstarman]

geodesic_distance has a Point overload and chord_distance did not, so the two verbs were usable at different levels. #720 flagged the asymmetry and left it open; this closes it.

It could not mirror geodesic_distance

That overload brings both operands into a Cartesian chart. For a chord that is exactly wrong:

• a Euclidean manifold is its own ambient, so chord_distance refuses it — converting to Cartesian would land every call on the refused case;
• an intrinsic sphere chart has no global Cartesian representation, so there is nothing to convert to. geodesic_distance(p, q) on sph2 raises NoGlobalCartesianChartError today for precisely that reason.

So the second operand is mapped into the first's chart instead, and the measurement delegated to the manifold-level rule. The chord is a property of the embedding, and the intrinsic chart is what carries it.

>>> p = cx.Point({"theta": u.Angle(jnp.pi/2, "rad"), "phi": u.Angle(0.0, "rad")}, chart=cxc.sph2)
>>> q = cx.Point({"theta": u.Angle(jnp.pi/2, "rad"), "phi": u.Angle(jnp.pi/2, "rad")}, chart=cxc.sph2)
>>> round(float(cx.chord_distance(p, q)), 6)
1.414214

Frame-strict, matching geodesic_distance, and refusing a cross-manifold pair.

Tests

Eleven: the analytic 2 sin(dphi/2) across five separations, symmetry, chart-invariance with the operands in different charts, that it differs from the geodesic (sqrt(2) against pi/2 — the point of having two verbs), flat space refusing with the error pointing at geodesic_distance, and the cross-frame refusal.

241 passed across tests/unit/vectors; nox -s precommit clean.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a Point-level overload for chord_distance so it can be called ergonomically at the same level as geodesic_distance, while preserving chord semantics (map the second operand into the first operand’s chart and delegate to the manifold-level rule), and enforcing frame/manifold strictness.

Changes:

• Register a Point, Point dispatch for chord_distance that (1) rejects cross-frame and cross-manifold pairs and (2) converts b into a’s chart before delegating to manifold-level chord_distance.
• Add a focused unit test module covering analytic correctness on S^2, symmetry, chart-invariance, Euclidean refusal, and cross-frame refusal.
• Ensure the new registration module is imported during vectors initialization.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/unit/vectors/test_chord_distance_point.py	Adds unit tests for the new Point overload behavior and key refusal/invariance properties.
src/coordinax/vectors/_src/register_chord_distance.py	Implements the Point overload dispatch for chord_distance with frame/manifold guards and chart conversion.
src/coordinax/vectors/_src/init.py	Imports the registration module so the dispatch implementation is loaded.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.71%. Comparing base (015e547) to head (7ba33bb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   96.71%   96.71%           
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  Files         268      269    +1     
  Lines        9125     9140   +15     
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+ Hits         8825     8840   +15     
  Misses        300      300           

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.