feat: an interface inspector and a demo gallery #90

What

An inspector built with blit, to debug the interfaces built with blit, like ImGui's metrics window. Beside it, a demo gallery: one window showing every widget, style and layout feature with the code that builds it, the reference people learn blit from, as ImGui's demo window is.

Acceptance

• Shows the frame's ids, claims, layers and bands, windows and dock tree, the hot, active and focused ids, state store entries, id collisions (feat!: hashed widget ids with an id stack #94), and draw list counts
• Hovering an entry outlines its rect on the surface
• Toggled from the app with one call, costing nothing when off
• A demo gallery window covering every public widget, grown as each widget lands, and shipped in demo/