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

CMakeLists.txt

@@ -172,6 +172,7 @@ set(COMMON_SOURCES
         ErrorHandler.cc
         SyscallWrappers.cc
         ErrorRecovery.cc
+        RegexGuard.cc
         TerminalInputHandler.cc
         TerminalRenderer.cc
         TerminalFrontend.cc
@@ -384,6 +385,7 @@ if (BUILD_TESTS)
             ErrorHandler.cc
             SyscallWrappers.cc
             ErrorRecovery.cc
+            RegexGuard.cc
             KKeymap.cc
             SwapRecorder.h
             OptimizedSearch.cc