Add Bar Gauge Graph Type

Add a graph type to Giraffe that is a variant on the gauges to support horizontal & vertical bar gauges. This will display a single stat value with a min and max value.

Add an option to the bar gauge to change color depending on the percentage.

Start by locating the existing gauge graph implementation and the entry points used to register graph types in Giraffe. Define the bar gauge's horizontal and vertical layouts, single-value min/max display, and percentage-based color option; done means both orientations and the color behavior are supported consistently.

Written by the indexing model from the issue text.