fix: keep sanitised text stable when it is truncated

The cut was marked with U+2026, which NFKC decomposes into three dots —
so re-sanitising a truncated string grew it by two characters and re-cut
it at a different word boundary. A title would quietly rewrite itself
every time the event was re-ingested, and the module promises
idempotency in its own docstring.

Append what normalisation would produce instead. The existing corpus
missed this because its only over-length entry has no space in its last
40%, so it happened to re-truncate to the identical string; the new test
uses prose.

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

src/ingest/sanitize.ts

@@ -149,16 +149,27 @@ function collapse(input: string): string {
 function truncate(input: string, max: number): string {
   if (input.length <= max) return input;
 
-  let cut = input.slice(0, max - 1);
+  let cut = input.slice(0, max - ELLIPSIS.length);
   // Never end on half a surrogate pair.
   if (/[\uD800-\uDBFF]$/.test(cut)) cut = cut.slice(0, -1);
 
   const lastSpace = cut.lastIndexOf(" ");
   if (lastSpace > max * 0.6) cut = cut.slice(0, lastSpace);
 
-  return `${cut.replace(/[\s,;:.–—-]+$/, "")}…`;
+  return `${cut.replace(/[\s,;:.–—-]+$/, "")}${ELLIPSIS}`;
 }
 
+/**
+ * Three dots, not U+2026.
+ *
+ * NFKC decomposes the ellipsis character into these three anyway, so a "…"
+ * appended here would grow by two characters on the next pass and re-cut the
+ * string at a different word boundary — breaking the idempotency this module
+ * promises. Writing what normalisation would produce keeps the second pass a
+ * no-op.
+ */
+const ELLIPSIS = "...";
+
 export interface SanitizeTextOptions {
   /** Hard cap; the result is never longer. Defaults to the summary cap. */
   maxLength?: number;