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.h

@@ -0,0 +1,22 @@
+// RegexGuard.h - keep std::regex from overflowing the stack
+//
+// libstdc++'s std::regex matcher recurses roughly once per character of a
+// match, so a pattern such as ".*" or "a+" on a line of a few tens of
+// thousands of characters (minified JS/JSON) overflows a default 8 MiB
+// stack and crashes the editor.
+#pragma once
+
+#include <cstddef>
+#include <functional>
+
+namespace kte {
+// Renderers highlight regex matches only on lines up to this many bytes;
+// longer lines are drawn without match highlighting.
+constexpr std::size_t kRegexRenderLineLimit = 10000;
+
+// 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);
+} // namespace kte