Complete backend validation, documentation, and the major release

[uermel]

Parent epic: #84

Implementation: #97, stacked on #95

Goal

Provide exact-commit packaging/release gates now, then complete backend and documentation validation during the final ecosystem cutover to stable.

Implemented release scope

• Make the test workflow reusable for an exact commit SHA.
• Run the locked stable-Python 3.11 through 3.14 matrix before release build/publish jobs.
• Use uv 0.12.4 and assert final Python interpreters.
• Build the wheel and source distribution once from the validated commit.
• Inspect artifact structure, all 32 entry points, and source-lockfile inclusion without duplicating Requires-Python or dependency-policy assertions.
• Use actions/upload-artifact@v7 and actions/download-artifact@v7.
• Upload and publish the same inspected artifacts without rebuilding.

Deferred final-cutover scope

• Complete live local/S3-compatible/Secure Shell/ML Croissant and portal validation once the surrounding ecosystem is migrated.
• Keep Server Message Block optional and non-gating.
• Complete README/changelog/ecosystem documentation immediately before v2.0 promotion to stable.
• Configure the final required checks and validate the published core/utility artifacts together.
• Complete the broader converter/input-layout coverage matrix recorded in the migration plan.

The deferred documentation/backend/coverage pass does not block review of #97.

Acceptance criteria

• The exact release commit passes the locked stable-Python matrix.
• The inspected artifacts are exactly the artifacts published.
• All 32 entry points are present in the built wheel.
• Final backend, documentation, ruleset, coverage, and published-artifact checks pass before stable promotion.
• ci: gate releases on validated artifacts #97 passes and merges after feat: delegate feature persistence to copick #95.