stack: Changelog-Eintrag geschichtet - Impact-Gruppierung, version regrade, Zusammenfassungspflicht (schliesst #95)

Files changed:
- CHANGES.md
- DEVELOPMENT.md
- VERSION
- instructions/dev/stack-dev/SKILL.md
- instructions/dev/version-parts.md
- tools/CONTRACT.md
- tools/README.md
- tools/chemenu/commands/run_budget.py
- tools/chemenu/commands/version_cmd.py
- tools/chemenu/tests/test_run_budget.py
- tools/chemenu/tests/test_version_cmd.py
- tools/chemenu/version.py

instructions/dev/stack-dev/SKILL.md

@@ -107,9 +107,14 @@ stack development happens in the origin repo instead (see AGENTS.md's routing li
    version and a changelog entry:
 
    ```bash
-   tools/wikitool version bump --patch --title "<what changed>"
+   tools/wikitool version bump --patch --title "<what changed>" --impact medium
    ```
 
+   `--impact high|medium|low` (default `medium`) grades this bump in the changelog entry's own
+   list - `tools/wikitool version regrade` corrects it later if the candidate's overall shape
+   changes the read on an earlier one; see
+   [instructions/dev/version-parts.md](../version-parts.md) § The candidate model.
+
    Never edit `VERSION` or the entry's heading by hand - `bump` writes both, and `docs verify`
    fails a tree where they disagree. Pick the part by whether the new version is a **drop-in
    replacement** for the old one - not by whether content has to be migrated: