M4: expose nested instances' properties on instances (#22)

- A nested instance inside a main component can be exposed (`component.exposeInstance`, an
  "Expose properties to instances" toggle in its panel); the main's instances then list it by
  name with its variant and component property controls, recursively for exposed instances
  inside it. Values set there are nested overrides and survive main edits.
- `instance.exposed {id}` lists an instance's exposed nested copies (for the CLI and MCP).

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/component.rs

@@ -77,6 +77,39 @@ pub fn specs() -> Vec<CommandSpec> {
                 Ok(json!({"id": i, "parent": slot.parent}))
             }
         ),
+        spec!(
+            "component.exposeInstance",
+            "Expose nested instance",
+            "{id? (an instance inside a main component), exposed? (default true)} — its properties then show on the main's instances",
+            true,
+            |s, p| {
+                let cmd = "component.exposeInstance";
+                let id = targets(s, p).first().copied().ok_or_else(|| bad(cmd, "select a nested instance"))?;
+                let exposed = boolean(p, "exposed").unwrap_or(true);
+                let doc = s.doc_mut()?;
+                let ok_target = doc.node(id).is_some_and(|n| matches!(n.kind, NodeKind::Instance { .. }))
+                    && owning_component(doc, id).is_some_and(|c| c != id)
+                    && !inside_instance(doc, id);
+                if !ok_target {
+                    return Err(bad(cmd, "only instances nested inside a main component can be exposed"));
+                }
+                doc.get_mut(id)?.exposed = exposed;
+                Ok(json!({"id": id, "exposed": exposed}))
+            }
+        ),
+        spec!(
+            "instance.exposed",
+            "Exposed nested instances",
+            "{id? (instance)} → the nested instance copies whose properties it exposes",
+            false,
+            |s, p| {
+                let id = targets(s, p).first().copied().ok_or_else(|| bad("instance.exposed", "select an instance"))?;
+                let doc = s.doc()?;
+                let out: Vec<serde_json::Value> =
+                    exposed_copies(doc, id).into_iter().filter_map(|c| doc.node(c).map(|n| json!({"id": c, "name": n.name}))).collect();
+                Ok(json!({"instances": out}))
+            }
+        ),
         spec!("component.detach", "Detach instance", "{ids?}", true, detach),
         spec!("component.resetInstance", "Reset all changes", "{ids?}", true, reset_instance),
         spec!(