Regex: optional PCRE2 engine behind kte::Regex

RegexEngine.h/.cc wrap the regex engine for search, regex replace-all and
the terminal and ImGui renderers' match highlighting:

- With KTE_USE_PCRE2 (default ON; used when pkg-config finds libpcre2-8,
  linked as -lpcre2-8) patterns are compiled by PCRE2 with its JIT and
  matched under a match limit: catastrophic backtracking stops with a
  status note instead of hanging the editor, and matching does not
  recurse on the C stack (no large-stack thread; the per-keystroke and
  per-frame line limit is 1 MiB instead of 2,000 bytes). Otherwise
  std::regex is used as before.
- Replacement strings follow std::regex_replace (ECMAScript as libstdc++
  reads it) with either engine: the PCRE2 path expands $&, $n, $`, $',
  $$ itself and steps over empty matches the same way.
- Regex search walks lines over the materialized text instead of calling
  GetLineView per row. The terminal renderer compiles the pattern once
  per frame (it compiled it for every row).
- 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 the buffer.
- default.nix and the Dockerfile add pcre2 and pkg-config.

On 26 MB: regex keystroke 1.6 -> 0.14 ms, next 62 -> 6 ms, replace-all
929 -> 438 ms; a rarely matching regex over 105 MB: 1.55 s -> 0.10 s.

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

Dockerfile

@@ -9,6 +9,8 @@ RUN apk add --no-cache \
     cmake \
     make \
     ncurses-dev \
+    pcre2-dev \
+    pkgconf \
     sdl2-dev \
     mesa-dev \
     freetype-dev \