fix: comparison and source pages accept the sources: cite add writes; sources may cite sources (#173)

Files changed:
- CHANGES.md
- VERSION
- instructions/wiki-ingest/SKILL.md
- kb/CONTRACT.md
- tools/CONTRACT.md
- tools/chemenu/commands/cite_cmd.py
- tools/chemenu/commands/xref.py
- tools/chemenu/tests/test_cite_cmd.py
- tools/chemenu/tests/test_page_ops.py
- tools/chemenu/tests/test_type_resolver.py
- tools/chemenu/tests/test_xref.py
- types/comparison.md
- types/comparison.schema.yaml
- types/source.md
- types/source.schema.yaml

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

instructions/wiki-ingest/SKILL.md

@@ -351,6 +351,10 @@ validator complains - and the ticked list is the only record that they happened.
    call of its own, is `kb/CONTRACT.md` § Linking. The second links the new source to everything
    it backs in one pass.
 
+   An older source page the new one cites does not go into `--entities`: `link-source` refuses
+   it. Record it as a citation instead, `tools/wikitool cite add --page "Source - <Title>"
+   --source "<older source>"` - `kb/CONTRACT.md` § Provenance and citation has the rule.
+
 10. **Check coverage.**
 
     ```bash