Remove Buffer::Rows() and the Line wrapper

The row cache rebuilt a string for every line of the file after each edit
and had one production caller left (filetype detection in the ImGui
frontend, which needs only the first line). Removed with it: rows_,
nrows_, the dirty flag set on every edit, ensure_rows_cache and a mutex
guarding only that cache. Tests read lines with GetLineString, Nrows or,
where they compare every line, a test-only LinesForTests().

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

tests/test_buffer_rows.cc

@@ -56,7 +56,7 @@ static void
 check_buffer_matches_model(const Buffer &b, const std::string &model)
 {
 	auto expected_lines = split_lines_preserve_trailing_empty(model);
-	const auto &rows    = b.Rows();
+	const auto &rows    = b.LinesForTests();
 	ASSERT_EQ(rows.size(), expected_lines.size());
 	ASSERT_EQ(b.Nrows(), rows.size());