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

RegexEngine.h

@@ -11,8 +11,11 @@
  * PCRE2 additionally has lookbehind, possessive quantifiers and the like.
  *
  * Replacement strings use ECMAScript syntax with either engine, as
- * std::regex_replace reads it: $& (the match), $1..$99 (groups), $` (the
- * text since the previous match), $' (the rest of the line), $$ (a dollar).
+ * std::regex_replace (libstdc++) reads it: $& or $0 (the match), $1..$99
+ * (groups, two digits read greedily; a group the pattern lacks expands to
+ * nothing), $` (the text since the previous match), $' (the rest of the
+ * line), $$ (a dollar sign). As in ECMAScript, '.' matches neither CR nor
+ * LF and '$' matches only at the end of the line's text.
  */
 #pragma once