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>

docs/mcp.md

@@ -419,6 +419,34 @@ and a file keeps the newest 20 autosaves (user versions are never dropped for th
   order: `{items: [{id, name, page, status}]}`. The Inspect tab sets the status of the selected
   frames and, with nothing selected, lists them with a status filter (click one to go to it).
 
+### Paint, effect and layout grid lists
+
+A layer's fills, strokes, effects and layout grids are lists kept **bottom first**: index 0 paints
+first, the last one is on top (the Design panel lists them the other way round, topmost first).
+
+- **Reorder:** `fill.move`, `stroke.move`, `effect.move` and `grid.move {ids?, from, to}` move one
+  item (one undo step for every layer given; out-of-range or equal indices change nothing, negative
+  or fractional ones are refused). The item keeps everything (an image fill its image, scale mode
+  and adjustments, a gradient its stops and handles), and a paint, effect or grid style linked to
+  the list is unlinked (the list no longer matches it). In the app, drag a row by its grip (the
+  empty strip left of its fields); a line shows where it lands.
+- **Variable bindings follow their items** when a list is reordered or an item removed
+  (`fill.remove`, `stroke.remove`, `effect.remove`, `grid.remove`); a removed item's bindings go
+  with it. Binding fields (`variable.bind {field}`): `fill` / `stroke` (the first paint's colour) or
+  `fills/<i>/color` / `strokes/<i>/color`, `effects/<i>/<offsetX|offsetY|radius|spread|color>`,
+  `layoutGrids/<i>/<size|count|gutter|offset>` (number variables; count is kept to 1–1000, size and
+  gutter are never negative). Moving a bound paint rewrites its key (`fill` becomes `fills/2/color`).
+- **Instance properties:** `variable.bind {ids: [instance], field: "componentProperties/<name>",
+  variable}` drives a boolean property from a boolean variable and a text property from a string
+  variable (other types are refused); the value follows the variable's modes like any binding, the
+  binding follows renames of the property, and deleting the property, resetting the instance or
+  detaching it drops it. The instance's properties panel offers the same with its variable button.
+
+```json
+{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"run_command","arguments":{"command":"fill.move","params":{"ids":[2],"from":0,"to":2}}}}
+{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"run_command","arguments":{"command":"variable.bind","params":{"ids":[5],"field":"layoutGrids/0/gutter","variable":"gutter"}}}}
+```
+
 ### Ruler guides
 
 `guide.add {vertical, offset, frame? | page?}` (returns `{index}`), `guide.move {index, offset, …}`,