Adapter node: IMAGE (batch) + BOUNDING_BOX (+ MASK, + LAYERS) -> LAYERS.
One document item per frame, each placed by its own box.
Why this is needed. A node that separates an image into elements emits
them as an image batch plus a list of boxes. That batch cannot be fed to
Create Layered Image with the placement intact, because
`expand_item_frames` applies the item's single x/y/w/h/name/z_index to
every frame of a batch. Sixteen layers get one placement between them.
The current workaround is to have the producer pre-place each layer on a
full-size canvas so x=0,y=0 is correct for all of them. That works but is
expensive: at 2K with 16 layers it carries ~768MB of layer tensors and
~268MB of masks to encode what cropped layers hold in a fraction of it.
This node emits one item per layer instead, so each carries its own
placement. It needs no change to the existing compositor path:
`document_items` already sorts by z_index, and `expand_item_frames`
already handles single-frame items correctly - the shared-placement
limitation only bites on batches.
Reads `metadata.name`, `metadata.z_index` and `metadata.content_rect`
where present. `crop_to_content` trims each frame out of a padded batch
and places it by its box, which is what recovers the memory win.
Also restores a `_bbox_list` parser. The equivalent (`_bbox_entries`,
`layout_bboxes`, `state_from_bboxes`) was removed in 1c4953ff along with
the rest of the bbox handling when the node moved to the LAYERS document,
so there is currently no path from a bounding box into the compositor.
Verified: two padded layers with different content sizes, cropped and
placed independently, composite to within 0.50/255 of expectation - the
8-bit quantisation floor of the PIL round trip inside the compositor.
The 76 existing compositor tests still pass.
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