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.md

@@ -4,7 +4,7 @@ name: comparison
 description: Structured type for comparison pages that set several entities or approaches against one another
 schema: types/comparison.schema.yaml
 base_dir: comparisons
-page_ref_fields: [entities, related]
+page_ref_fields: [entities, related, sources]
 guidance: types/comparison.guidance.md
 ---
 
@@ -24,6 +24,7 @@ and how to write a conforming page is [types/comparison.guidance.md](comparison.
 | `created` | Yes | Creation date (YYYY-MM-DD) |
 | `entities` | Yes | Titles of the entities compared |
 | `related` | No | Declared outbound edges - one `compares-with` edge per subject, written by `wikitool xref add` |
+| `sources` | No | Titles of the source pages cited on this page, written by `wikitool cite add` |
 | `summary` | Yes | One-liner for `kb/index.md` |
 
 ## Template