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.cc

@@ -49,21 +49,4 @@ RunWithLargeStack(const std::function<void()> &fn)
 		std::rethrow_exception(job.error);
 }
 
-
-void
-ForEachRegexMatch(const std::string &line, const std::regex &rx,
-                  const std::function<void(std::size_t, std::size_t)> &on_match)
-{
-	auto scan = [&] {
-		for (auto it = std::sregex_iterator(line.begin(), line.end(), rx); it != std::sregex_iterator(); ++it)
-			on_match(static_cast<std::size_t>(it->position()), static_cast<std::size_t>(it->length()));
-	};
-	// Stack use grows with match length; a few hundred bytes stays far below
-	// any thread's stack for every pattern we measured.
-	constexpr std::size_t kInlineLimit = 512;
-	if (line.size() <= kInlineLimit)
-		scan();
-	else
-		RunWithLargeStack(scan);
-}
 } // namespace kte