The contain branch chained a zoom resize into the contain resize on one Sharp pipeline; Sharp keeps a single set of resize options, so per-cell zoom was silently discarded and pan was equally dead. Splitting the chain would not have fixed it: the contain step scales the zoomed image straight back down.
The backend now implements the preview's semantics exactly: contain-fit, scale about the cell centre by zoom, offset by pan as a fraction of the cell, clip, background behind. Defaults keep the old single contain resize (now PNG-encoded so JPEG inputs no longer flatten cornerRadius masks to black corners), and the general path centres with floor to match Sharp's contain placement.
Nine marker-geometry tests, seven mutants each killed by the test built for them, window math fuzzed over 2.59M combinations.
Fixes#711
