MCP: Add a tool to change the stacking order of shapes

Summary
New shapes are always placed on top. There is no tool to bring a shape to the front or send it to the back.

Use case
To highlight a table row, I add a rectangle behind the existing text. Because it lands on top, it covers the text. Today I have to recreate the text on top of the highlight.

Proposal

reorder_shape shapeId: string position: "front" | "back" | "forward" | "backward" 

Alternatively, an optional index within the parent's children.