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

CMakeLists.txt

@@ -197,6 +197,7 @@ set(COMMON_SOURCES
         ErrorRecovery.cc
         RegexGuard.cc
         RegexEngine.cc
+        SearchHighlight.cc
         TerminalInputHandler.cc
         TerminalRenderer.cc
         TerminalFrontend.cc
@@ -296,6 +297,7 @@ set(COMMON_HEADERS
         TestFrontend.h
         TextSpan.h
         RegexEngine.h
+        SearchHighlight.h
         UndoNode.h
         UndoTree.h
         UndoSystem.h
@@ -413,6 +415,7 @@ if (BUILD_TESTS)
             ErrorRecovery.cc
             RegexGuard.cc
             RegexEngine.cc
+            SearchHighlight.cc
             KKeymap.cc
             SwapRecorder.h
             OptimizedSearch.cc