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

types/comparison.schema.yaml

@@ -38,6 +38,13 @@ properties:
       provenance-style list, so without this one the edge the page exists to
       make would live only in hand-written prose - which is the thing
       instructions/link-taxonomy.md was built to end.
+  sources:
+    type: array
+    items:
+      type: string
+    description: >-
+      Source page titles cited on this page, written by `wikitool cite add`.
+      Optional, so a comparison that cites nothing keeps validating without it.
   summary:
     type: string
     description: 1-line summary for index.md