add Genshin Impact Fandom event source

Add genshin-impact.fandom.com as a second source for Genshin Impact,
which has been fixture-backed in CI because Game8 CloudFront-blocks the
Actions runner with 202s.

- Add fifth Fandom template parser in src/ingest/parsers/fandom.ts
  extracting titles from caption display text, stripping dated subpage
  suffixes, and checking currency via latestBoundaryMs.
- Register genshin-fandom-events in src/ingest/adapters/index.ts and
  set genshin-game8-events priority: 10 so existing event IDs survive
  near-match ties, preventing localStorage completion marks from
  orphaning.
- Explicitly sort adapter groups by priority descending in
  scripts/build-feed.ts to align with parseGame.
- Pin fixtures and add test coverage in test/adapters/game8.test.ts.
- Update AGENTS.md, README.md, docs/INGESTION.md, and docs/SOURCES.md.

README.md

@@ -20,7 +20,7 @@ review queue are specified but not built.
 | Piece | State |
 |---|---|
 | Event schema, date parsing, Game8 parser | Built, tested |
-| Twenty sources across nineteen games | Built, tested |
+| Twenty-one sources across nineteen games | Built, tested |
 | Your own games and events, one-off or repeating | Built, tested |
 | Cross-source merge and conflict detection | Built, tested |
 | Input sanitization at the ingest boundary | Built, tested |
@@ -111,7 +111,7 @@ date makes you miss content. Given the choice, this ships nothing rather than a
 
 | Game | Source |
 |---|---|
-| Genshin Impact | Game8 |
+| Genshin Impact | Game8 + Fandom |
 | Honkai: Star Rail | Game8 |
 | Wuthering Waves | Game8 |
 | Zenless Zone Zero | Game8 |