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]>

test/merge.test.ts

@@ -191,3 +191,50 @@ describe("merging two sources for one game", () => {
     expect(conflicts[0]?.field).toBe("endsAt");
   });
 });
+
+describe("near matches within one source", () => {
+  test("two rows from one page stay two events", () => {
+    // The page has already told us these are two things, and the parser has
+    // already dropped repeats of the same id — so fusing them here overrules a
+    // distinction the publisher made on purpose. Game8's Umamusume banner list
+    // is the real case: two concurrent banners whose titles differ by one
+    // parenthetical and which start on the same day. Fusing them dropped a live
+    // banner off the calendar with nothing reporting it.
+    const character = event({
+      id: "uma:3-star-guaranteed-1-5-anniversary-scout-character:2026-07-22",
+      game: "uma",
+      title: "3 Star Guaranteed 1.5 Anniversary Scout (Character)",
+      sourceId: "uma-game8-events",
+    });
+    const support = event({
+      id: "uma:3-star-guaranteed-1-5-anniversary-scout-support:2026-07-22",
+      game: "uma",
+      title: "3 Star Guaranteed 1.5 Anniversary Scout (Support)",
+      sourceId: "uma-game8-events",
+    });
+
+    const merged = mergeEvents([[character, support]]);
+    expect(merged.events).toHaveLength(2);
+    expect(merged.conflicts).toHaveLength(0);
+  });
+
+  test("the same id twice from one source is still one event", () => {
+    // The exception that keeps the rule safe: an identical id is the same row
+    // seen twice, whatever it is called.
+    const once = event({ sourceId: "source-a" });
+    const twice = event({ sourceId: "source-a", title: "Test Event (again)" });
+    expect(mergeEvents([[once, twice]]).events).toHaveLength(1);
+  });
+
+  test("near matches across two sources still fuse", () => {
+    // The behaviour this rule narrows, not removes: reconciling two sources
+    // that describe one event under different titles is the whole job.
+    const a = event({ title: "Bedazzling Dawnstar", sourceId: "source-a" });
+    const b = event({
+      id: "genshin:bedazzling-dawnstar-sign-in:2026-08-12",
+      title: "Bedazzling Dawnstar Sign-In",
+      sourceId: "source-b",
+    });
+    expect(mergeEvents([[a], [b]]).events).toHaveLength(1);
+  });
+});