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

tools/CONTRACT.md

@@ -755,12 +755,14 @@ Batch-link a source page to every entity/concept it mentions.
 - 0    success
 - 1    Source page not found
 - 1    A page in `--entities` does not exist, or a target could not be written; the others were linked
+- 1    A page in `--entities` is itself a source page; the others were linked
 - 1    The source page itself could not be written after its targets were
 
 **ON FAILURE**
 
 - Source page not found -> Fix the title and retry once
 - A page in `--entities` does not exist, or a target could not be written; the others were linked -> Fix the named pages and re-run - safe, since every write is idempotent. `sources trace --page "<Title>"` shows who is already linked
+- A page in `--entities` is itself a source page; the others were linked -> Drop it from `--entities` and record the citation with `cite add --page "<citing source>" --source "<cited source>"` instead
 - The source page itself could not be written after its targets were -> Fix the write failure and re-run (idempotent)
 
 **NOTES**
@@ -769,6 +771,7 @@ Batch-link a source page to every entity/concept it mentions.
 - Which field a target lands in follows its collection (`kb/entities/` -> `entities:`), so a new collection needs no code change.
 - A target whose collection matches no reference field the source type declares is linked one way and named in the output.
 - A target that does not exist is skipped and named; the others are still linked, and the run then exits 1.
+- A target that is itself a source page is refused, named with the `cite add` command to use instead, and handled like a missing page: nothing is written for it, the others are still linked, and the run exits 1. `link-source` means "A mentions X" and writes `X.sources += A`, which for X = another source would claim the citation in the wrong direction; a citation between two sources has a direction only `cite add` records.
 - Idempotent in both directions. `--dry-run` reports what would be linked.
 
 **SEE ALSO**
@@ -897,7 +900,8 @@ Upsert a `[^cite-id]: [[Source - X]]` definition in a page's footnotes region.
 
 - Upserts a `[^cite-id]: [[Source - X]]` definition in the page's generated footnotes region, creating the region between `<!-- wikitool:footnotes -->` markers if absent.
 - Reuses the id when the page already cites this exact source/file pair, so a repeat changes nothing.
-- Adds `Source - X` to the page's frontmatter `sources:`.
+- Adds `Source - X` to the page's frontmatter `sources:` - except on `Source - X` itself: a source page citing one of its own raw files (`--file`) gets the definition and leaves `sources:` untouched, since a page never lists its own title there.
+- A source page may cite another source page this way - `cite add` is the command that records a citation between two sources, in the direction it was made.
 - Prints the `[^cite-id]` marker; pasting it into the prose is a manual, editorial step.
 - `--dry-run` reports without writing.