A controller is Xema's equivalent of a Kubernetes controller: a deterministic loop that takes the current state of a set of objects and drives it toward the desired state. Controllers are how the platform stays consistent under concurrent writes and partial failures — they reconcile, they don't fire-and- forget. Controllers may be shipped by platform services (apps/) or by biome manifests via the controller contribution kind. Each controller declares the kinds it watches, the events it emits, and the environment it runs in.