Render: add backend capability discovery and strict request resolution (#650)

Adds render.backends, a metadata-only command that lists the render backends, their executors and profiles, with strict request resolution in the render crate (docs/render-backends.md).

docs/features.md

@@ -481,6 +481,12 @@ SolidCraft's commands by area, generated by `cargo xtask features` from the comm
 | Insert Part | `parts.insert` | – |
 | Change Part Size | `parts.resize` | – |
 
+## `render.*` (1)
+
+| Command | Id | Toolbar |
+|---|---|---|
+| Render Backends | `render.backends` | – |
+
 ## `select.*` (2)
 
 | Command | Id | Toolbar |