55032546 (release commit body) and d1022f8b (lockstep bump) shipped without
the per-version CHANGELOG heading or the README version row. publish.yml's
'Extract release notes' step requires a '## <version> — <date>' heading in
root CHANGELOG.md; without it, a v4.0.52 tag push fails fast at that gate.

This commit lands the missing pieces in lockstep with the bumped root
version 4.0.52:

- CHANGELOG.md: add '## 4.0.52 — 2026-09-17 (...)' with 6 highlights
  (silent-false-pass index axis, repair-index execution-proven, slice-003
  apply, user-reported kind vocabulary + parser-syntax follow-ups, 12+
  defect sweep, vitest.workers single source) plus a verification block
  and an explicit 'not validated as done' block for the rollback contract
  of repair-index and the still-fail-silent refreshCodegraphAfterSlice.
- README.md / README-en.md: 4.0.49 -> 4.0.52 (2026-09-17) via
  scripts/sync-readme-version.mjs (which reads the date from the
  CHANGELOG heading so the two files never disagree).