refactor(timeline): extract entryKey helper from YearBand

Move the per-entry {#each} key logic into a shared entryKey.ts so the
undated bucket in TimelineView can reuse it. No behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/timeline/YearBand.svelte

@@ -4,6 +4,7 @@ import WorldBand from './WorldBand.svelte';
 import LetterCard from './LetterCard.svelte';
 import YearLetterStrip from './YearLetterStrip.svelte';
 import { isDense } from './timelineDensity';
+import { entryKey } from './entryKey';
 import type { components } from '$lib/generated/api';
 
 type TimelineYearDTO = components['schemas']['TimelineYearDTO'];
@@ -42,16 +43,6 @@ const rows = $derived.by<Row[]>(() => {
 	}
 	return out;
 });
-
-function entryKey(entry: TimelineEntryDTO): string {
-	return (
-		entry.kind +
-		':' +
-		(entry.eventId ??
-			entry.documentId ??
-			`${entry.derivedType}:${(entry.linkedPersonIds ?? []).join('-')}`)
-	);
-}
 </script>
 
 <section class="py-2">

frontend/src/lib/timeline/entryKey.ts

@@ -0,0 +1,23 @@
+import type { components } from '$lib/generated/api';
+
+type TimelineEntryDTO = components['schemas']['TimelineEntryDTO'];
+
+/**
+ * Stable `{#each}` key for a timeline entry. Prefers the entry's own identity
+ * (`eventId` for curated events, `documentId` for letters); derived life-events
+ * carry neither, so they key on `derivedType` + their linked person ids — which
+ * keeps two derived births in the same year distinct. The `kind` prefix keeps an
+ * event and a letter that happen to share an id from colliding.
+ *
+ * Used by both `YearBand` (per-band rows) and `TimelineView` (the undated
+ * bucket), where entries can be events without a `documentId`.
+ */
+export function entryKey(entry: TimelineEntryDTO): string {
+	return (
+		entry.kind +
+		':' +
+		(entry.eventId ??
+			entry.documentId ??
+			`${entry.derivedType}:${(entry.linkedPersonIds ?? []).join('-')}`)
+	);
+}