feat: Versionsstelle als Kompatibilitaetsfrage, Breaking-Change-Vermerk erzwungen (2.5.0)

Files changed:
- CHANGES.md
- INSTALL.md
- VERSION
- instructions/dev/stack-dev/SKILL.md
- instructions/dev/version-parts.md
- tools/CONTRACT.md
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/commands/version_cmd.py
- tools/chemenu/tests/test_docs_verify.py
- tools/chemenu/tests/test_version_cmd.py
- tools/chemenu/version.py

instructions/dev/stack-dev/SKILL.md

@@ -48,6 +48,9 @@ stack development happens in the origin repo instead (see AGENTS.md's routing li
    [testing-conventions.md](../testing-conventions.md) - the suite runs against a deliberately
    empty machine; what the autouse fixture already neutralizes, and what a test still has to
    establish itself. Read it before adding or changing a test.
+   [version-parts.md](../version-parts.md) - which part a change bumps: the drop-in test, the
+   catalogue of breaks that cross the compatibility boundary with `kb/` untouched, and what to
+   put in front of the user before a breaking bump. Read it before step 3.
    More instructions are added here incrementally as stack-development needs come up - this
    list grows without needing this skill file to change shape.
 3. **Raise the version, if the change ships.** A change under `tools/`, `types/`,
@@ -59,18 +62,27 @@ stack development happens in the origin repo instead (see AGENTS.md's routing li
    ```
 
    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 what an existing instance would have to do:
+   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:
 
    | Change | Part |
    |--------|------|
    | Fix, no interface change | `--patch` |
-   | New capability, backwards compatible | `--minor` |
-   | **Existing content must be migrated** | `--major` |
+   | New capability, still drop-in in both directions | `--minor` |
+   | **Not a drop-in replacement** - any hand-work by the user or a migration script, or a downgrade that no longer works | `--major` |
 
-   A `--major` bump additionally needs a migration document for the new version - written per
-   [migrate-corpus.md](../../migrate-corpus.md) - or `--no-migration "<reason>"` when no content
-   actually has to change. `bump` refuses otherwise, and so does `docs verify`: an instance
-   learning that it must migrate, with nothing telling it how, is a dead end.
+   Content migration is one way to land in the last row, not the definition of it: a rename of
+   the update path, the artefact, an import name, a flag or an envvar breaks a swap with `kb/`
+   entirely untouched. The full test, the catalogue of such breaks, and what to put in front of
+   the user first are in [version-parts.md](../version-parts.md) - **read it before choosing
+   `--major`.**
+
+   A `--major` bump therefore needs two things recorded. `--breaking "<what stops working>"`
+   is required on every boundary-crossing bump; on top of it, a migration document for the new
+   version - written per [migrate-corpus.md](../../migrate-corpus.md) - or
+   `--no-migration "<reason>"` when no content actually has to change. `bump` refuses without
+   either, and so does `docs verify`: an instance learning that it must migrate, with nothing
+   telling it how, is a dead end.
 
    Then write the entry's body - `bump` deliberately leaves it empty, the same way `new` leaves
    the prose.
@@ -90,6 +102,12 @@ stack development happens in the origin repo instead (see AGENTS.md's routing li
 - **Touches both stack code and wiki content in one session?** Apply this skill's rules to the
   code changes and the normal content skills' rules to the content changes - they are not
   mutually exclusive within a session, only per change.
+- **The change turns out not to be a drop-in replacement?** Do not bump across the boundary on
+  your own initiative. Every existing instance pays for a breaking change once, by hand, so the
+  user decides whether it is worth that: show them what breaks, what an instance has to do about
+  it, and the alternatives (avoid the break with a shim, defer and batch it with the next one,
+  or split it behind a deprecation window), then recommend one and wait for a go-ahead.
+  [version-parts.md](../version-parts.md) step 4 has the full shape.
 
 ## Scope