Fix review findings in the regex engine, search count and journal v2

- PCRE2: compile with the ANYCRLF newline convention and DOLLAR_ENDONLY,
  as ECMAScript reads '.' and '$'. With PCRE2's default (LF only) '.'
  matched the CR of a CRLF line, so regex replace-all deleted CRs.
- Replacement $0 is the whole match, two digits are read greedily and a
  group the pattern lacks expands to nothing, as std::regex_replace
  (libstdc++) does; these cases are now compared in the tests.
- Plain search visits overlapping matches going forward too, and counts
  them, as the backward search found them ('aa' in 'aaa' read 2/1).
- The recovery prompt's replayed content is used on 'y' only if the
  journal is unchanged since; otherwise it is replayed again.
- A version 1 journal continued after recovery is rewritten as a
  compacted version 2 journal when it needs a chunked checkpoint,
  instead of receiving records an older kte would skip. (The version is
  read through a separate descriptor: the journal fd is write-only.)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

docs/audits/claude-findings-20260923.md

@@ -558,6 +558,15 @@ before the fix (unless noted).
   1.55 s -> 0.10 s per keystroke.
 - Only one checkpoint per journal is queued at a time; whole-file edits
   in a row on a large buffer each queued a full copy of it.
+- Review of this work found, and these are fixed: with PCRE2, '.'
+  matched the CR of CRLF lines (regex replace-all deleted it) — PCRE2
+  now uses the ANYCRLF newline convention and $ matches only at the end;
+  $0, out-of-range $n and $nn in replacements now follow libstdc++;
+  plain search visits overlapping matches in both directions (the count
+  could read "2/1"); the recovery prompt replays again if the journal
+  changed while it was open; a version 1 journal continued after
+  recovery gets a chunked checkpoint only by being rewritten as
+  version 2.
 
 **Known limitations (not fixed)**
 - Without PCRE2 (std::regex builds), catastrophic regex backtracking