Establish the copick-utils migration test and packaging gate

[uermel]

Parent epic: #84

Implementation: #94, based directly on v2.0

Goal

Establish a reproducible pull-request test and packaging gate before changing the runtime stack.

Implemented scope

• Add a test optional dependency and committed uv.lock.
• Add a pull-request workflow covering stable Python 3.11 through 3.14.
• Run pull-request checks for every changed path so documentation-only PRs still report required status; retain path filtering only for pushes.
• Use uv 0.12.4 and assert that the selected interpreter is a final release.
• Add deterministic numeric-path and s0 OME-Zarr fixtures.
• Capture the current chunk-wise feature result with nonconstant, non-divisible, multi-chunk data.
• Record the pre-migration feature-store incompatibility until feat: delegate feature persistence to copick #95 delegates persistence to core.
• Allow the workflow to validate an exact release SHA for reuse by ci: gate releases on validated artifacts #97.

Tests are behavior-focused. There are no AST/source-inventory or dependency-string tests. Broader direct-reader and converter coverage remains deferred.

Acceptance criteria

• The baseline behavior suite passes from a clean locked installation.
• The feature compatibility fixture remains reproducible.
• Every pull request, including documentation-only changes, runs the stable Python matrix.
• The workflow can be called with an exact release commit.
• test: establish Zarr migration baseline #94 passes and merges before the next stack layer.