M3: reorder paints, effects and layout grids in the Design panel; grid and instance property variable bindings (#81)

Fill, stroke, effect and layout grid rows drag to reorder by their grip (a handle shows over the
row, an insertion line where it lands, the row's label follows the pointer like a dragged layer;
Escape cancels; lists that differ across the selection have nothing to drag). New commands
`fill.move`, `stroke.move` and `grid.move {ids?, from, to}` join `effect.move` (lists bottom first,
one undo step; items keep everything, linked list styles are unlinked).

Variable bindings on list items follow their items when a list is reordered or shrinks:
paint colours bind per paint (`fills/<i>/color`, `strokes/<i>/color`; `fill`/`stroke` stay the
first paint's), so a moved bound paint keeps its variable and its neighbours are never recoloured.
`fill.remove`, `stroke.remove` and `grid.remove` refuse negative indices.

Layout grids get settings in the Design panel (size; count, type, width or height, margin or
offset, gutter; colour) whose numbers bind to number variables (`layoutGrids/<i>/<field>`),
re-applied on variable and mode changes. Instance boolean and text properties bind to boolean and
string variables (`componentProperties/<name>`, from the instance's properties panel), kept across
instance rebuilds, following property renames and dropped on delete, reset and detach.
Inspect names the variable of each paint.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -9,6 +9,7 @@ pub mod file;
 pub mod image;
 pub mod layout;
 pub mod library;
+pub mod lists;
 pub mod page;
 pub mod props;
 pub mod proto;
@@ -67,6 +68,7 @@ fn registry() -> &'static Registry {
         let mut list = Vec::new();
         list.extend(create::specs());
         list.extend(edit::specs());
+        list.extend(lists::specs());
         list.extend(arrange::specs());
         list.extend(structure::specs());
         list.extend(section::specs());