Share search-match highlighting across renderers; engine owns its stack policy

SearchHighlight compiles the search pattern once per frame and yields each
line's match ranges; the terminal, ImGui and Qt renderers had three copies
of this, and Qt compiled a std::regex for every visible line on every
frame. Qt now uses kte::Regex like the others.

kte::Regex::RunGuarded replaces Command.cc's run_regex_work, which chose
the large stack by comparing engine names. ForEachRegexMatch had no
callers left and is removed.

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

RegexGuard.h

@@ -6,31 +6,13 @@
 // stack and crashes the editor.
 #pragma once
 
-#include <cstddef>
 #include <functional>
-#include <regex>
-#include <string>
 
 namespace kte {
-// 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;
-
 // 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
 // cannot overflow. If such a thread cannot be created, fn runs on the calling
 // thread. Exceptions thrown by fn are rethrown to the caller.
 void RunWithLargeStack(const std::function<void()> &fn);
 
-// Call on_match(position, length) for each match of rx in line. Lines short
-// enough to be safe on any stack are matched inline; longer ones on a large
-// stack (a 10,000-byte line already overflowed 8 MiB for some patterns).
-// Throws what std::regex throws.
-void ForEachRegexMatch(const std::string &line, const std::regex &rx,
-                       const std::function<void(std::size_t, std::size_t)> &on_match);
 } // namespace kte