feat: insert another design as a linked component (#596)

component.insert_design adds a component holding another design's solid bodies, linked to its file (stored relative to the assembly once saved). component.update_links reads the linked designs again, component.break_link keeps the bodies and forgets the file, and component.links lists them. Links are saved under format 16; an older build keeps the bodies and drops the link.

Fixes #533

crates/engine/src/cmd/mod.rs

@@ -22,6 +22,7 @@ mod face;
 mod features;
 mod features_more;
 pub use features::auto_operation;
+mod component_link;
 mod config;
 mod file;
 mod inspect;
@@ -204,6 +205,7 @@ pub fn command_specs() -> Vec<&'static CommandSpec> {
     v.extend(sheet::COMMANDS.iter());
     v.extend(plastic::COMMANDS.iter());
     v.extend(component::COMMANDS.iter());
+    v.extend(component_link::COMMANDS.iter());
     v.extend(joints::COMMANDS.iter());
     v.extend(motion::COMMANDS.iter());
     v.extend(config::COMMANDS.iter());