fix(merge): fuse near matches only across sources

Near matching exists to reconcile two sources describing one event under
different titles. Within one source it has no such job: the page has already
told us these are two rows, the parser has already dropped repeats of the same
id, and fusing them overrules a distinction the publisher made on purpose.

The loose 0.80 threshold that makes the rule useful across sources makes it
actively wrong within one. Umamusume's banner list runs "3 Star Guaranteed 1.5
Anniversary Scout (Character)" and "(Support)" concurrently — titles differing
by a single parenthetical, starting the same day — and fusing them dropped a
live banner off the calendar with nothing anywhere reporting it. A silent drop
is the failure this codebase ranks as the dangerous one.

Same-id fusion is untouched, so nothing duplicates.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -119,7 +119,7 @@ src/client/       React app, service worker, manifest
                   theme.ts — dark or light, and what a game hue reads as on each
 scripts/          build-feed.ts, build-static.ts, parse-fixture.ts (offline), refresh-sources.ts (fetches)
 serve.ts          static server + /api/health
-test/             683 tests
+test/             686 tests
 fixtures/<game>/  raw HTML + .expected.json per source — pinned, kept forever
 snapshots/        current page per source, rewritten by refresh — see its README
 ```