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]>

docs/INGESTION.md

@@ -338,9 +338,10 @@ Only meaningful when a game has more than one source; a single-source game passe
 
 `mergeEvents(groups)` compares events across sources:
 
-1. **Same ID** → same event; keep the higher-confidence copy.
-2. **Near match** — same game, title similarity ≥ 0.80, starts within 24h — → same event under
-   different titles; keep the higher-confidence copy.
+1. **Same ID** → same event; keep the higher-confidence copy. Applies within a source as well as
+   across sources — an identical id is the same row seen twice, whatever it is called.
+2. **Near match** — **different sources**, same game, title similarity ≥ 0.80, starts within 24h —
+   → same event under different titles; keep the higher-confidence copy.
 3. **Otherwise** → distinct events; keep both.
 
 Title similarity alone would merge a rerun with its original, since reruns reuse the name. The
@@ -348,6 +349,23 @@ start-date proximity check is the actual guard; the title threshold is deliberat
 that "Stygian Onslaught" and "Stygian Onslaught Event" collapse into one row rather than showing
 the user a duplicate.
 
+**Near matching is cross-source only, and that restriction is load-bearing** (2026-08-19). Fusing
+two rows from *one* page overrules a distinction the publisher made on purpose, and the loose
+threshold that makes rule 2 useful across sources makes it actively wrong within one: Game8's
+Umamusume banner list runs `3 Star Guaranteed 1.5 Anniversary Scout (Character)` and `(Support)`
+concurrently, titles differing by a single parenthetical and starting the same day. That scores far
+above 0.80, and fusing them dropped a live banner off the calendar with nothing anywhere reporting
+it — a silent drop, which § Silent drops ranks as the dangerous failure. Rule 1 still fuses repeats
+within a source, so nothing is duplicated; the parsers dedupe by id before merge is reached anyway.
+
+**Agreement raises confidence (+0.10) only across different `sourceId`s.** The same row seen twice
+in one document is not corroboration.
+
+**Disagreement is surfaced, never averaged.** Two sources whose `endsAt` differ by more than 24
+hours produce a `conflicts` entry; the pipeline routes those to quarantine. Splitting the difference
+between two dates would produce a value neither source asserts — the worst possible answer for a
+product whose promise is date accuracy.
+
 ## Stage 4 — validate
 
 Zod parse against `GachaEvent`, then calendar sanity rules. Anything failing a hard rule goes to