ci(release): build the GitHub Release body from the CHANGELOG section

[lance0]

Problem

The release job (release.yml) created the GitHub Release with generate_release_notes: true only — GitHub's bare auto PR-list — and never read CHANGELOG.md. So every release shipped thin notes until someone hand-patched the body with the curated ## [X.Y.Z] section. (v0.5.0 only looked right because I set it by hand during the ship.)

Fix

Add an Extract release notes from CHANGELOG step that pulls the section between the tag's ## [<version>] heading and the next ## [ (validated locally against the 106-line 0.4.0 section — all 5 subsections, no truncation), and feed it as body_path. Keep generate_release_notes: true so GitHub appends its "What's Changed" PR list + full-changelog link below the curated notes — best of both.

Falls back to auto-notes with a ::warning:: if the tag has no CHANGELOG entry.

Notes

• Takes effect on the next release; v0.5.0's notes are already correct.
• Closes the "auto-populate the GitHub Release body from the CHANGELOG" roadmap item. A hard CHANGELOG-presence check that fails a tag (vs. warns) is noted as an optional follow-up — it needs its own tag-triggered job since ci.yml only runs on branch pushes.
• No Rust changed; YAML validated (yaml.safe_load).