Highlighter engine returns cached spans by reference; drop unneeded mutexes

HighlighterEngine::GetLine copied the span vector on every call (twice
on a miss) and did a dynamic_cast per call; it now returns the cached
entry (valid until the next call) and dispatches on Stateful().

PieceTable and HighlighterEngine took a mutex in their read paths while
their writes took none, and PieceTable.h claimed concurrent reads were
safe; they are not (reads update caches). Every Buffer is used from the
main thread only (the swap writer thread receives copies; the regex
large-stack thread runs while its caller waits), so the mutexes are
removed and the headers say so. ASan and the threaded tests under TSan
are clean.

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

QtFrontend.cc

@@ -308,7 +308,7 @@ protected:
 					// as operators).
 					if (buf->SyntaxEnabled() && buf->Highlighter() && buf->Highlighter()->
 					    HasHighlighter()) {
-						kte::LineHighlight lh = buf->Highlighter()->GetLine(
+						const kte::LineHighlight &lh = buf->Highlighter()->GetLine(
 							*buf, static_cast<int>(i), buf->Version());
 						struct SSpan {
 							std::size_t s;