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/source.schema.yaml

@@ -72,6 +72,15 @@ properties:
     items:
       type: string
     description: Concept titles mentioned in this source
+  sources:
+    type: array
+    items:
+      type: string
+    description: >-
+      Other source pages this source cites, written by `wikitool cite add`.
+      Never this page's own title - a citation of one of its own raw files
+      leaves the field untouched. Optional, so a source that cites no other
+      source keeps validating without it.
   summary:
     type: string
     description: 1-line summary for index.md