lib/changelog.sh gains changelog_fragments (publication order: trailing
issue number descending, filename tie-break), changelog_fragment_problem
(the release-time rules moved onto the PR that writes the fragment, #112
D9), and changelog_assemble (canonical group order per D5, one shape per
repo per D4). bin/changelog-assemble folds changelog.d/ into one release
section, deletes exactly what it consumed, and --check proves the body
without touching the tree.
Part of #112. Closes#114 groundwork; tests follow.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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