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

Swap.h

@@ -237,6 +237,7 @@ private:
 		bool gap{false};
 		bool gap_notified{false}; // the user was told about the current gap
 		bool chkpt_queued{false}; // a checkpoint is queued and not yet being written
+		std::uint32_t header_version{0}; // of the open journal file (0: none yet)
 		bool lockout_notified{false}; // told about the lock; kept across ResetJournal
 		std::uint64_t gap_chkpt_request_ns{0};
 		bool gap_unfixable_reported{false}; // gap on a buffer too large to checkpoint