Rect: split into Heatmap and Histogram

Currently the "rect" layer type can be both a Heatmap or a Histogram. While that works for now, it will be harder to maintain when Heatmap and Histogram need their own improvements and updates.

Split the `Rect` component into two separate components to allow Heatmap and Histogram to diverge independently.

Start by locating the Rect component and the code paths that represent Heatmap and Histogram layers. Trace how the current rect layer selects between them, then separate those responsibilities so Heatmap and Histogram can evolve independently; done means both layer types still work through their own components.

Written by the indexing model from the issue text.