Return buffered execution_output chunks for a live cell run.

Used by the frontend on reconnect/reload to recover any chunks it missed while disconnected from the notebook socket room. Authorized by the same notebook-lock check as /execute: the caller must own the lock on the handle's notebook.

after_seq lets clients page incrementally — pass the highest seq they've already rendered (live or replayed) and only newer chunks are returned. When after_seq is 0 or predates the buffer, chunks is empty and snapshot carries the reduced outputs as of snapshot.seq, which replace the cell's outputs. Returns 404 when the per-cell replay record has been pruned; the cell's final cell.outputs is then the source of truth.