Problem: Mesh Warp keeps its Distortion Mesh in a hidden text parameter,
and the viewer only draws handles for point parameters (Bezier Warp's),
so with Mesh Warp selected there was no grid and nothing to drag: the
effect could not warp anything from the UI.
Fix: with a Mesh Warp effect (or one of its parameters) selected and
layer controls shown, the viewer draws the (rows+1)x(columns+1) grid
over the layer in effect space, with a handle on every vertex
(automation id viewer.meshWarp.<fx>.<row>.<col>). Dragging a vertex
writes its offset to the `mesh` parameter through prop.set, one undo
step per drag; a mesh that does not match Rows/Columns starts from the
undistorted grid, as it renders.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>