Add base instances for instance config inheritance

Instances can be marked as a base with instance.is_base and other
instances inherit config values from it through instance.base_id.
The controller copies base values into inheriting configs and keeps
track of inherited fields in the hidden instance.inherited_fields, so
hosts receive resolved configs and need no changes. Setting a field
stops inheriting it, `instance config inherit` undoes that.

Bases cannot be assigned to a host, are hidden from `instance list`
unless --bases is given, and cannot be deleted or unmarked while
other instances inherit from them. Chains of bases are allowed and
cycles are rejected.

Part of #200

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/controller/Controller.js

@@ -34,6 +34,10 @@ class MockInstanceConfig extends EventEmitter {
 		this._data.set(field, value);
 		this.emit("fieldChanged", field, value, prev);
 	}
+
+	isInherited() {
+		return false;
+	}
 }
 
 describe("controller/src/Controller", function() {