read_batch_item trusted list_items(offset=position) to land on position.
That only holds while item rows are dense: OFFSET-based paging skips
surviving rows over any gap (e.g. a future retention delete), so the
reader would serve the next item's report attributed to the requested
item key, and the tool layer would project next_position from the wrong
base. The browser projection (read_browser_batch_item) already verifies
items[0].position; align the model-facing reader with it and add the
same negative-position guard. Reading past a gap now returns not_found
instead of a shifted row, matching the browser path's semantics.
