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

RegexGuard.h

@@ -12,13 +12,14 @@
 #include <string>
 
 namespace kte {
-// Renderers highlight regex matches only on lines up to this many bytes, and
-// search-as-you-type skips longer lines (an explicit search does not):
+// Used where std::regex is still called directly (the Qt frontend's match
+// highlighting; search and the other renderers use RegexEngine.h, whose
+// Regex::IncrementalLineLimit() applies the same limits for std::regex).
+// Regex matches are highlighted only on lines up to this many bytes:
 // std::regex can take time quadratic in line length even for ordinary
 // patterns (".*q" on a 20 KB line: 12 s), with no way to interrupt it, and
 // these run on every frame or keystroke.
-constexpr std::size_t kRegexRenderLineLimit      = 2000;
-constexpr std::size_t kRegexIncrementalLineLimit = 2000;
+constexpr std::size_t kRegexRenderLineLimit = 2000;
 
 // Run fn to completion on a thread with a large stack (address space is
 // reserved, pages are committed only as used), so regex work over long lines