Plugin UI rendering adds an Image element alongside Raster

What

• The rendering diff/restamp pipeline now handles a new Image element type alongside the existing Raster, with matching stamp/seen-key/refusal logic and an error when a render hook returns an image it didn't draw.
• The Image render-props validator now accepts an optional key prop, which must be a non-empty string when present, and for images without columns/rows or a resolvable source, it requires the image to be "stamped by a plugin of this chain," otherwise reporting Image "<key>" is not stamped by a plugin of this chain.
• The raster/paint tool's input can now target an Image source directly, using { requestId, key, source }, in addition to the existing cell-data form { requestId, key, cells }.
• ui.blit change detection now also treats a change in whether a source field is present or absent as meaningful, and its description now mentions "the Raster or Image being blitted" instead of just the Raster.

Why

This lets plugins draw with a full image source rather than only an encoded grid of cells, while keeping the same ownership and stamping checks that stop one plugin from tampering with another plugin's drawing.