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

tests/test_audit_regressions.cc

@@ -24,6 +24,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 #include <random>
+#include <regex>
 #include <set>
 #include <vector>
 #include <clocale>
@@ -888,17 +889,20 @@ TEST(Audit_Commands_FilesystemErrorsDoNotThrow)
 }
 
 
-// Renderer regex highlighting on a long line with a deeply recursive pattern
-// (overflowed an 8 MiB stack at under 10,000 bytes) runs off the main stack.
-TEST(Audit_ForEachRegexMatch_LongLine)
+// std::regex on a long line with a deeply recursive pattern (overflowed an
+// 8 MiB stack at under 10,000 bytes) completes on the large stack.
+TEST(Audit_RunWithLargeStack_LongLine)
 {
 	std::string line;
 	while (line.size() < 200000)
 		line += "word, more words; and so on. ";
 	std::size_t n = 0, total = 0;
-	kte::ForEachRegexMatch(line, std::regex("(\\w|\\s|[.,;])+"), [&](std::size_t, std::size_t len) {
-		++n;
-		total += len;
+	const std::regex rx("(\\w|\\s|[.,;])+");
+	kte::RunWithLargeStack([&] {
+		for (auto it = std::sregex_iterator(line.begin(), line.end(), rx); it != std::sregex_iterator(); ++it) {
+			++n;
+			total += static_cast<std::size_t>(it->length());
+		}
 	});
 	ASSERT_EQ(n, (std::size_t) 1);
 	ASSERT_EQ(total, line.size());