M8.6: Puppet Warp pins appear at once, warp from the rest shape and follow Undo

- Pins show as soon as the tool has art selected: automatic ones at the
  centre and the middle of each limb's end (at least three), until pins are
  placed. They stay across tool switches while the selection is unchanged and
  a new selection starts afresh.
- The pins live in the document (Document.puppet, never saved) with the rest
  shape they started from: every warp starts from it, so warps don't stack and
  dragging a pin back restores the original; Undo/Redo take the pins back with
  the art. Adding and deleting pins are undo steps too.
- object.puppetWarp gains rest: true (the tool's pins); the new query
  object.puppetWarp.pins reads them. A click off the mesh adds nothing and
  says so (not-allowed cursor, canvas label); the command refuses such pins.
- The mesh grid covers the expanded art, so tiny art with a wide Expand no
  longer allocates millions of cells.
- Tests on groups, compound paths and open strokes.

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

crates/engine/tests/command_sweep.rs

@@ -279,6 +279,10 @@ fn structured_junk() {
         ("object.blend.spine.moveAnchor", json!({"id": 2, "anchor": u64::MAX, "x": 1e308, "y": -1e308, "handle": "sideways"})),
         ("object.blend.spine.addAnchor", json!({"id": u64::MAX, "x": 0, "y": 0})),
         ("object.blend.spine.removeAnchor", json!({"anchor": -1})),
+        ("object.puppetWarp", json!({"rest": true, "pins": [], "moved": []})),
+        ("object.puppetWarp", json!({"rest": true, "pins": [[1e308, 0]], "moved": [[0, 0]], "expand": -5})),
+        ("object.puppetWarp", json!({"rest": true, "ids": [u64::MAX], "pins": [[0, 0]], "moved": [[0, 0]]})),
+        ("object.puppetWarp.pins", json!({"ids": [0, 1, 2], "expand": 1e308})),
     ];
     let mut failures = vec![];
     for fx in Fixture::ALL {