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).

crates/engine/src/cmd/mod.rs

@@ -38,6 +38,7 @@ pub(crate) use measure_sel::measure_items;
 mod kernel_heal_tests;
 mod parameters;
 mod plastic;
+mod render;
 mod sheet;
 mod sketch;
 pub(crate) use sketch::author_path_for_rebind;
@@ -222,6 +223,7 @@ pub fn command_specs() -> Vec<&'static CommandSpec> {
     v.extend(section::COMMANDS.iter());
     v.extend(view_navigation::COMMANDS.iter());
     v.extend(diagnostics::COMMANDS.iter());
+    v.extend(render::COMMANDS.iter());
     v
 }