♻️ refactor(transforms): declare groups() on AbstractTransform

[nstarman]

Stacks on #728 — review the top commit only, or merge #728 first. It removes the ty: ignore that PR had to add.

The gap

Every concrete transform defines groups(), and callers depend on it — Composed.groups reads it off each component, _merge off both operands — but AbstractTransform declared nothing. It was a protocol by convention, with nothing on the base to point at.

A type checker could therefore only see the attribute where the caller happened to hold a concrete type. ty passes Composed.groups merely because its transforms field is loosely typed, and fails the moment a parameter is annotated AbstractLinearTransform — which is exactly what #728 hit, and what its ty: ignore was covering.

The fix

One abstract method on the base. Spelled as an instance method, so both existing forms satisfy it:

• most operators answer from their type alone and implement it as a classmethod, which stays reachable from an instance
• Composed and Linear compute it from what they hold

Abstract rather than a default. All eleven concrete transforms already implement it:

HAVE   : Boost, Composed, Identity, Linear, LorentzBoost, Reflect,
         Rotate, Scale, Shear, TimeDep, Translate
MISSING: AbstractAdd, AbstractCompositeTransform, AbstractLinearTransform

The three that don't are the abstract intermediates, which is what abstract means. A default returning some fallback group would be a wrong answer waiting for a subclass that forgot to override — and since groups() feeds least_common_supergroup, a wrong group silently widens every fusion that touches it.

Verification

• Full suite: 10391 passed, 10 skipped, 1 xfailed
• prek run --all-files clean, including ty with ✨ feat(transforms): add Linear, so mixed linear chains fuse #728's suppression removed

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR strengthens the transforms API by formally declaring groups() on AbstractTransform, making group membership an explicit part of the base interface while supporting both class-derived and instance-computed implementations. It also expands simplification/fusion capabilities for linear transforms by introducing a general Linear operator and adding additional _merge rules with accompanying tests.

Changes:

• Add an abstract groups() method to AbstractTransform to formalize group membership as part of the core transform interface.
• Introduce Linear (a general linear transform that carries its group as data) and a generic _merge(AbstractLinearTransform, AbstractLinearTransform) fallback for mixed linear fusion.
• Add a _merge(Scale, Scale) rule and extend simplify tests to cover new scale merging and mixed linear fusion behavior.

Reviewed changes

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

Show a summary per file

File	Description
tests/unit/transforms/test_simplify.py	Adds tests for Scale merging and mixed linear fusion into Linear, including group propagation assertions.
src/coordinax/transforms/_src/actions/scale.py	Adds _merge(Scale, Scale) to fuse adjacent scales via matrix multiplication.
src/coordinax/transforms/_src/actions/general_linear.py	Adds the new Linear transform, its group-carrying behavior, simplify rule, and generic linear _merge fallback.
src/coordinax/transforms/_src/actions/base.py	Declares AbstractTransform.groups() as an abstract method with documentation and example.
src/coordinax/transforms/_src/actions/init.py	Exports the new general_linear action module.
src/coordinax/transforms/init.py	Re-exports Linear as part of the public coordinax.transforms API.

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[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.55%. Comparing base (cd59977) to head (f1db6a2).

Files with missing lines	Patch %	Lines
...oordinax/transforms/_src/actions/general_linear.py	95.45%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #730   +/-   ##
=======================================
  Coverage   96.54%   96.55%           
=======================================
  Files         265      266    +1     
  Lines        8780     8828   +48     
=======================================
+ Hits         8477     8524   +47     
- Misses        303      304    +1     

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