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

kb/CONTRACT.md

@@ -300,7 +300,8 @@ Every claim is either traceable to a raw file or explicitly marked as not.
   [--file <qualifier>]` mints the id, upserts its `[[Source - X]]` (or
   `[[Source - X|storage-model.md]]` for a multi-file source) definition in the page's trailing
   Footnotes block (named per [Section headings](#section-headings)), and adds `Source - X` to
-  `sources:` - it prints the marker to paste at
+  `sources:` - unless the page *is* `Source - X`, citing one of its own raw files, since a page
+  never lists its own title there. It prints the marker to paste at
   the fact; placing it is still manual. Never hand-type a cite-id (AGENTS.md invariant 1). This
   differs from a plain `[[Source - X]]` link, which only means "related to".
 - **Notation inside code is notation, not a reference.** A `[^cite-id]` or a `[[wikilink]]`
@@ -309,13 +310,19 @@ Every claim is either traceable to a raw file or explicitly marked as not.
   means a marker appended to a line *inside* a fence cites nothing - put it on a source line
   under the block (`<source-word>: [^cite-id]`, in the KB language), where it renders as a
   footnote instead of travelling with the command when someone copies it.
-- A source cited inline must also appear in the page's frontmatter `sources:` list;
-  `wikitool lint` checks this in both directions, and hard-errors on a leftover pre-migration
+- **A source page may cite another source page**, the same way any page does: `cite add --page
+  "Source - A" --source "Source - B"` writes `Source - B` into A's `sources:` and leaves B
+  untouched. A citation between two sources has a direction, and `cite add` is the only command
+  that records it - `xref link-source` refuses a target that is itself a source page.
+- A source cited inline must also appear in the page's frontmatter `sources:` list - except a
+  source page's citation of itself, which never does; `wikitool lint` checks this in both
+  directions, and hard-errors on a leftover pre-migration
   `^[[...]]` marker, an undefined `[^cite-id]` reference, or an orphaned Footnotes definition.
   `tools/wikitool cite sync` reconciles a page's block after a prose edit changes which ids are
   actually referenced.
 - `tools/wikitool xref link-source --source "Source - X" --entities A,B,C` adds a new source
-  to every page it backs in one pass.
+  to every page it backs in one pass. Its targets are the pages the source *mentions*, never
+  another source page.
 - Every raw file is expected to be claimed by some source page;
   `tools/wikitool sources coverage` lists the ones that are not.