Adopt a conflict-free CHANGELOG workflow for parallel PRs

[DocGerd]

Problem

The current changelog ritual (#131) has every user-visible-behavior PR add an entry to CHANGELOG.md's single [Unreleased] section. This repo routinely develops several PRs in parallel (parallel implementer agents). When 2+ of those each edit the same [Unreleased] region, they conflict — a merge conflict, or a forced re-sync collision under the strict up-to-date policy on develop.

(Config/tooling/docs-only PRs correctly get no changelog entry, so they don't contribute to this — the conflict is specifically between multiple user-visible-behavior PRs.)

Options (pick one)

1. Changelog fragments — recommended, industry best practice. Each PR adds a small, uniquely-named file under changelog.d/ (e.g. changelog.d/165.fixed.md) and never edits CHANGELOG.md directly, so two PRs never touch the same file → zero conflicts. A release step (towncrier / changesets / a tiny local script) assembles fragments into the release section at cut time and deletes them. Atomic with each PR and conflict-free.
2. Interim rule, no tooling. Until fragments land: when 2+ PRs run in parallel, do NOT have each edit CHANGELOG.md. Add the entry only in the last PR of the batch, or open a single dedicated changelog PR at batch/session end. A solo PR may keep its atomic entry (no conflict possible).

Acceptance criteria

• Decide fragments vs. interim rule.
• If fragments: add changelog.d/ + an assembly step, wired into the release cut / Release-cut documentation sweep (do with the next develop→main release) #132 docs sweep and the About-dialog "What's new" build (CHANGELOG.md (Keep a Changelog) + changelog visible in the app #131); delete fragments on assembly.
• Update the CLAUDE.md changelog ritual to encode the chosen rule.
• Update the PR-template CHANGELOG checkbox (Add PR template codifying the CLAUDE.md merge ritual #172) accordingly, plus CONTRIBUTING.
• Verify no regression to the in-app "What's new" changelog view.

References

• CHANGELOG.md (Keep a Changelog) + changelog visible in the app #131 (changelog ritual + in-app view), Release-cut documentation sweep (do with the next develop→main release) #132 (release-cut docs sweep), Add PR template codifying the CLAUDE.md merge ritual #172 (PR template).
• Prior art: towncrier (pip/pytest), Changesets (JS monorepos).