[SIMPL-30451] Add distributed-execution service with guide and reference implementations

Canonical location for documentation, example workflows and reference service implementations covering distributed execution patterns. Covers AC1-AC4: execution-target selection, decision support, readiness checks and code-level linkage. Tightly coupled jobs and the loosely coupled subprocess transport are verified by the test suite; the Kubernetes pipes transport is implemented but not yet cluster-run and is marked as such in the guide.

Changelog: added

yaml/loosely-coupled/rbac-pipes-dispatch.yaml

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+# Kubernetes RBAC for the loosely coupled execution target.
+#
+# Granted to the service account running the Dagster RUN pods - the pod that
+# calls PipesK8sClient.run() is the one that creates and watches the payload Job.
+#
+# Note what is NOT here: the payload itself needs no RBAC, no database
+# credentials and no Vault role. Its only channel back to the control plane is
+# the pod log stream, which the dispatching pod reads.
+#
+# If the payload runs in a different namespace from the dispatcher, apply this
+# Role in the PAYLOAD namespace and keep the RoleBinding subject pointing at the
+# dispatcher's service account.
+
+apiVersion: rbac.authorization.k8s.io/v1
+kind: Role
+metadata:
+  name: distributed-execution-pipes-dispatcher
+  namespace: dagster
+rules:
+  - apiGroups: ["batch"]
+    resources: ["jobs", "jobs/status"]
+    verbs: ["create", "get", "list", "watch", "delete"]
+  # pods/log is the message channel for PipesK8sPodLogsMessageReader.
+  # Without it the payload runs but reports nothing.
+  - apiGroups: [""]
+    resources: ["pods", "pods/log", "pods/status"]
+    verbs: ["get", "list", "watch"]
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: distributed-execution-pipes-dispatcher
+  namespace: dagster
+subjects:
+  - kind: ServiceAccount
+    name: dagster-dev
+    namespace: dagster
+roleRef:
+  kind: Role
+  name: distributed-execution-pipes-dispatcher
+  apiGroup: rbac.authorization.k8s.io