New 'Raster' component lets plugins paint to a terminal canvas

What

A new Raster element type is available for plugins to use in their UI. It takes props like {key, columns, rows, cells}, where cells is base64-encoded packed cell data, and plugins can repaint it via a paintRaster-style call ({ requestId, key, cells, columns, rows }).

• The rendering layer now sizes and lays out Raster elements like other UI primitives, and it participates in existing press/hosted-button handling.
• Global UI state now tracks mountedRasters (which rasters are currently mounted) and rasterAdmission (whether a raster is allowed to render).
• Behind the scenes it includes an SGR-color cache, per-second blit (draw) throttling, and validation limited to BMP images with single-width cells.

Why

This gives plugin authors a way to draw arbitrary pixel-like graphics into the terminal UI, beyond the text-based components available before, while the admission/throttling logic keeps repeated repaints from overwhelming the terminal.