Automate release notes with changelog fragments

[azimafroozeh]

What & why

Unrelated changes currently edit the same release document. Add permanent changelog.d/ fragments and generate release notes from new files since an explicit previous release.

The generator produces versioned snapshots, updates the release index and supplies the GitHub release body. Documentation CI provides a preview for review.

Backing issue / RFC

Not linked.

Checklist

• One logical change: release-note authoring and generation.
• Regression tests added for behavior changes.
• Contributor and release documentation updated.
• Maintainer review against docs/dev/invariants.md.

Local verification

Python checks used an isolated environment with the pinned documentation dependencies.

• scripts/test_release_notes.py: 48 tests passed.
• scripts/check-docs.py: 161 Markdown files checked.
• Applicable workflow guards, YAML parsing, spelling and whitespace checks passed.

Notes for reviewers

• Notes remain in Git after release. Validation detects changes to fragments present in the configured previous release.
• Snapshots are checked against the audited release tree, including after squash merges. Committed preview links use the selected source SHA.
• The existing v0.12.0 note body is preserved as a one-time baseline. Markdown validation uses pinned CommonMark dependencies.
• Snapshot and index replacement is atomic per file. An interrupted index update is detected and recoverable with index --write.

Release publication was not run locally.

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