M4.88: Object → Slice makes, edits and lays out user, object and auto slices

Document.slices holds user slices, Node.slice makes an object an object
slice that follows its bounds, and Document.slices_clip_to_artboard picks
the region auto slices fill (the artboards, or the art and the slices).
Object → Slice: Make, Release, Create from Guides, Create from Selection,
Duplicate, Combine, Divide… (dialog), Delete All, Slice Options… (dialog)
and Clip to Artboard; View → Hide/Lock Slices; Select → Object → Slices;
slice.list reads the numbered layout. The canvas draws the slices in the
Slices preferences' line colour with their numbers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -47,6 +47,7 @@ pub mod prefscmds;
 pub mod rasterfx;
 mod recolor;
 mod select;
+pub(crate) mod slices;
 mod stroke;
 mod style;
 pub mod stylelib;
@@ -225,6 +226,7 @@ pub fn command_specs() -> &'static [CommandSpec] {
         v.extend(pdfcmds::specs());
         v.extend(fileio::dxf::specs());
         v.extend(package::specs());
+        v.extend(slices::specs());
         v
     })
 }