fix(#3001): indent continuation lines in serializeChangelog (#3101)

* fix(#3001): indent continuation lines in serializeChangelog

serializeChangelog interpolated bullet bodies verbatim into a single
- ${body} (#${pr}) line. Any embedded newline became a column-0 line;
parseChangelog's continuation-fold (/^\s+/) didn't pick it up, so
flushBullet terminated the bullet early — dropping the continuation text
and the (#NNNN) PR trailer (recorded as pr: null). The round-trip property
serialize(IR) → parse(text) === IR was false for any body containing \n.

Fix: indent continuation lines (body.replace(/\n/g, '\n  ')) so the
parser folds them correctly. Round-trip test asserts both paragraphs'
content AND the PR number survive.

* chore(#3001): backfill changeset PR number 3101

---------

Co-authored-by: sim <sim@local>

scripts/changeset/serialize.cjs

@@ -26,7 +26,11 @@ function serializeChangelog(ir) {
     lines.push(`### ${section.type}`);
     lines.push('');
     for (const b of section.bullets) {
-      lines.push(`- ${b.body} (#${b.pr})`);
+      // #3001: indent continuation lines so parseChangelog's continuation-fold
+      // (/^\s+/) picks them up instead of terminating the bullet at the first
+      // blank line. A multi-paragraph body round-trips with content preserved.
+      const body = b.body.replace(/\n/g, '\n  ');
+      lines.push(`- ${body} (#${b.pr})`);
     }
     lines.push('');
   }