Journal insert_row as one record; keep the cursor in the buffer; fix ENABLE_ASAN

- Buffer::insert_row recorded its text and newline as two swap records
  after inserting both. A checkpoint triggered by the first (1 MiB of
  edits, or the checkpoint timer) snapshotted the newline already, and
  the second added it again on replay: recovery after undoing C-k C-k
  silently gained a blank line. It is now one record.
- Visual-line yank placed the cursor relative to its row assuming that
  row was inside the selection; after C-k a or Enter it was not, and the
  cursor ended past the end of a line or the buffer, where typed text was
  recorded for undo at a position it was not inserted at and could not
  be undone. The formula uses the nearest selected row, and run_handler
  now clamps the cursor to the buffer before and after every command.
- ENABLE_ASAN added -fsanitize=address to CMAKE_C_FLAGS only; the
  project is C++, so nothing was instrumented. Add it to
  CMAKE_CXX_FLAGS.

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

Buffer.cc

@@ -929,10 +929,14 @@ Buffer::insert_row(int row, const std::string_view text)
 	rows_cache_dirty_ = true;
 	edited_at_(row);
 	if (swap_rec_) {
-		// Avoid allocation: emit the row text insertion (if any) and the newline insertion.
-		if (!text.empty())
-			swap_rec_->OnInsert(row, 0, text);
-		swap_rec_->OnInsert(row, static_cast<int>(text.size()), std::string_view("\n", 1));
+		// One record: the first of two could trigger a journal checkpoint,
+		// whose snapshot already holds the newline the second then added
+		// again on replay.
+		std::string line;
+		line.reserve(text.size() + 1);
+		line.append(text);
+		line.push_back('\n');
+		swap_rec_->OnInsert(row, 0, line);
 	}
 }