Regex: don't overflow the stack on long lines

libstdc++'s std::regex matcher recurses about once per matched
character, so '.*', 'a+' or '(a|b)*' on a line of a few tens of
thousands of characters (minified JS/JSON) overflowed the 8 MiB stack
and crashed the editor from the regex search prompt or regex replace.

Regex search and regex replace now run their matching on a worker
thread with a 1 GiB reserved stack (RunWithLargeStack; the caller
waits, so nothing is shared concurrently). Renderers skip regex match
highlighting on lines over 10,000 bytes. Catastrophic backtracking
(e.g. (a*)*b) is still possible: std::regex has no time limit.

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

RegexGuard.cc

@@ -0,0 +1,51 @@
+#include "RegexGuard.h"
+
+#include <exception>
+#include <pthread.h>
+
+namespace kte {
+namespace {
+// 1 GiB supports matches spanning several million characters.
+constexpr std::size_t kLargeStackBytes = std::size_t{1} << 30;
+
+struct Job {
+	const std::function<void()> *fn;
+	std::exception_ptr error;
+};
+
+
+void *
+run_job(void *arg)
+{
+	auto *job = static_cast<Job *>(arg);
+	try {
+		(*job->fn)();
+	} catch (...) {
+		job->error = std::current_exception();
+	}
+	return nullptr;
+}
+} // namespace
+
+
+void
+RunWithLargeStack(const std::function<void()> &fn)
+{
+	Job job{&fn, nullptr};
+	pthread_attr_t attr;
+	bool started = false;
+	pthread_t tid{};
+	if (pthread_attr_init(&attr) == 0) {
+		if (pthread_attr_setstacksize(&attr, kLargeStackBytes) == 0)
+			started = pthread_create(&tid, &attr, run_job, &job) == 0;
+		pthread_attr_destroy(&attr);
+	}
+	if (!started) {
+		fn();
+		return;
+	}
+	pthread_join(tid, nullptr);
+	if (job.error)
+		std::rethrow_exception(job.error);
+}
+} // namespace kte