[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/values-pipes-payload.yaml

@@ -0,0 +1,32 @@
+# Loosely coupled execution target - code location overrides.
+#
+# Merge into the dagster-user-deployments entry for this code location. These
+# values point the pipes client at the payload image and target namespace.
+#
+# Contrast with yaml/tightly-coupled/values-run-launcher.yaml: there, every
+# execution pod needed S3 and Vault credentials. Here the payload needs none -
+# only the dispatching pod does.
+
+dagster:
+  dagster-user-deployments:
+    deployments:
+      - name: distributed-execution
+        envSecrets: []
+        env:
+          - name: PIPES_PAYLOAD_IMAGE
+            value: code.europa.eu:4567/simpl/simpl-open/data/supporting-data-services/distributed-execution/payload:0.1.0
+          - name: PIPES_PAYLOAD_NAMESPACE
+            value: dagster
+
+  # The dispatching RUN pod creates the payload Job, so the image reference and
+  # namespace must also be visible to run pods, not just the code server.
+  runLauncher:
+    config:
+      k8sRunLauncher:
+        runK8sConfig:
+          containerConfig:
+            env:
+              - name: PIPES_PAYLOAD_IMAGE
+                value: code.europa.eu:4567/simpl/simpl-open/data/supporting-data-services/distributed-execution/payload:0.1.0
+              - name: PIPES_PAYLOAD_NAMESPACE
+                value: dagster