Clicking a Layers panel target circle runs `layer.target {id}`: a layer gets its art
selected and becomes the target (`Selection.target`) of the appearance, effect,
transparency and opacity-mask commands, which then edit the layer itself; the
Appearance panel lists it. Dragging a circle onto another runs
`appearance.transfer {source, target, copy?}` (Alt copies), dropping it on the trash
clears it. Filled dots also mark groups and layers with fills of their own, masked
objects get a dashed underline, and while an opacity mask is edited the panel lists
only an `<Opacity Mask>` entry and the tab says so.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
layer.clippingMask.toggle {id?} -> {clip}: the top object of the current
layer (or of the one selected group, or of `id`) becomes the clipping path,
unpainted and moved to the bottom so art added later is clipped too; pressing
the button again releases the mask and leaves the path unpainted. One undo
step. The Layers panel button runs it and underlines clipping-path names.
Model: NodeKind::Layer.clip (skipped when false). Clipped layers render, hit
test, bound, and export to SVG and PDF like clip groups (Node::clips /
set_clips); the clip-path helpers are shared with Make/Release Clipping Mask.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
