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

RegexEngine.cc

@@ -2,6 +2,8 @@
 
 #include <regex>
 
+#include "RegexGuard.h"
+
 #if defined(KTE_USE_PCRE2)
 #define PCRE2_CODE_UNIT_WIDTH 8
 #include <pcre2.h>
@@ -223,6 +225,13 @@ Regex::LimitHit() const
 }
 
 
+void
+Regex::RunGuarded(const std::function<void()> &fn)
+{
+	fn();
+}
+
+
 const char *
 Regex::EngineName()
 {
@@ -311,6 +320,13 @@ Regex::LimitHit() const
 }
 
 
+void
+Regex::RunGuarded(const std::function<void()> &fn)
+{
+	RunWithLargeStack(fn);
+}
+
+
 const char *
 Regex::EngineName()
 {