✨ feat(distances): promote AbstractDistance against ParametricQuantity

[nstarman]

What

The core half of #672. ParametricQuantity is not a unxt.Q subclass, so the AbstractDistance/Q rules in distances/_src/base.py never reached it:

Distance(1, "pc") * PQ(1.0, "rad")  ->  ValueError: Distance must have dimensions length.
PQ(1.0, "rad") * Distance(1, "pc")  ->  PQ(1., 'pc rad')

The operand order decided whether the expression worked. #672 fixed this for coordinaxs.astro's Parallax and DistanceModulus; coordinax.distances.Distance was left with the same defect.

How

• distances/_src/register_parametric.py — the one add_promotion_rule, and the only file importing unxts.parametric.
• Imported from distances/_src/__init__.py behind OptDeps.UNXTS_PARAMETRIC.installed.
• coordinax/_src/optional_deps.py — first Python use of optional-dependencies in core, which was already a declared dependency. Floor moves >=0.3.2 → >=0.5.0, where members sharing a value no longer silently alias.
• parametric = ["unxts.parametric>=2.0"] extra.

Overlap with #672

Astro's rules are on Parallax/DistanceModulus, which are AbstractDistance subclasses, so this rule subsumes them. Theirs stay: astro must work against a coordinax predating this, and where both apply plum takes the more specific one. Confirmed the two coexist — all three types give a ParametricQuantity in both operand orders.

Values are unchanged, only reachability. PQ * Distance is PQ(1., 'pc rad') both with and without the new rule; the difference is that Distance * PQ now returns it instead of raising.

Verification

• tests/: 2153 passed, 6 skipped
• packages/coordinaxs.astro: 398 passed, 2 skipped (coexistence)
• Both gate paths exercised: with the distribution hidden from importlib.metadata, coordinax.distances imports without pulling in unxts.parametric, and the new tests skip rather than fail
• pre-commit (incl. ty): clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

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Adds an optional promotion rule so AbstractDistance interoperates consistently with unxts.parametric.ParametricQuantity, fixing operand-order-dependent behavior and wiring the registration behind an optional dependency gate.

Changes:

• Register a Plum promotion rule between AbstractDistance and ParametricQuantity when unxts.parametric is installed.
• Introduce a small internal optional-dependency enum (OptDeps) and bump optional-dependencies floor to support it safely.
• Add test coverage to verify promotion + order-independent arithmetic under the optional install.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/unit/distances/test_parametric_promotion.py	Adds tests exercising the optional registration and ensuring Distance * PQ works.
src/coordinax/distances/_src/register_parametric.py	Defines the AbstractDistance ↔ ParametricQuantity promotion rule.
src/coordinax/distances/_src/init.py	Conditionally imports the parametric registration based on OptDeps.
src/coordinax/_src/optional_deps.py	Adds internal OptDeps enum wrapper around optional-dependencies.
pyproject.toml	Raises optional-dependencies minimum version and adds a parametric extra.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.12%. Comparing base (71b9ec7) to head (09532d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #679   +/-   ##
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  Coverage   96.12%   96.12%           
=======================================
  Files         254      254           
  Lines        8179     8182    +3     
=======================================
+ Hits         7862     7865    +3     
  Misses        317      317           

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