Measure which mask layers a crop takes out of the frame

Mask geometry is stored in source coordinates, so re-cropping tighter
never destroys a layer. It makes it invisible: the layer stays in the
panel and the sidecar, its adjustment lands on pixels nobody will see,
and nothing says so. The spec had no clause for this; FR-DEV-17 now
states it, under the ID issue #10 reserved.

dr_pipeline::orphan samples each layer's mask on a 64x64 lattice over
the source, with the gradient, radial, brush and model-raster geometry
the mask shader uses, folds the parts by their joins and inversions, and
maps the samples through the framing to see how much of the coverage
the crop keeps. `hidden_by_crop` reports the layers whose share fell
below a tenth, and only those the change newly hid, so an already
stranded layer is not announced again on every later adjustment.

Ranges follow the picture and region selections need a label map this
crate does not hold, so a layer that adds either is never reported: a
false alarm on the common path would teach the notice to be dismissed
unread.

core/dr-pipeline/src/lib.rs

@@ -44,6 +44,7 @@ pub mod mask;
 pub mod neutral;
 pub mod operation;
 pub mod ops;
+pub mod orphan;
 pub mod preset;
 pub mod sidecar;
 pub mod spot;