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

QtFrontend.cc

@@ -1,4 +1,5 @@
 #include "QtFrontend.h"
+#include "RegexGuard.h"
 
 #include <QApplication>
 #include <QWidget>
@@ -165,7 +166,9 @@ protected:
 						    (ed_->CurrentPromptKind() == Editor::PromptKind::RegexSearch ||
 						     ed_->CurrentPromptKind() ==
 						     Editor::PromptKind::RegexReplaceFind)) {
-							try {
+							// Long lines are not highlighted: std::regex recursion
+							// could overflow the stack (RegexGuard.h).
+							if (line.size() <= kte::kRegexRenderLineLimit) try {
 								std::regex rx(ed_->SearchQuery());
 								for (auto it = std::sregex_iterator(
 									     line.begin(), line.end(), rx);