[SIMPL-30451] Close the distributed execution reference service

Check the chart's actual DAGSTER_PG_PASSWORD injection in the tightly coupled preflight instead of DAGSTER_POSTGRES_*, which the Simpl chart never sets; the old check warned about a misconfiguration on a correctly configured run pod. Evidence reports the variable name only, never its value.

Record the platform k8s_job_executor run as cluster-verified in the guide and readiness checklist, add the Dagster UI screenshots covering job list, graph, code-configured tags and a successful run.

Changelog: fixed

CHANGELOG.md

@@ -9,10 +9,25 @@
   (`PipesSubprocessClient`, `PipesK8sClient`) with a standalone external payload image.
 - Working example configuration and RBAC for both execution targets.
 
+### verified
+
+- `tightly_coupled_k8s_job` executed end to end on sandbox-cat-dat through the
+  platform `K8sRunLauncher` (run `1d8cb167-9fb8-4a34-a036-789eac381e13`), closing
+  readiness rows T6 and T7. Step outputs are written to the shared
+  `dagster-shared-pvc` by pinning `fs_io_manager`'s `base_dir`; `DAGSTER_HOME` set
+  through `step_k8s_config` is ignored, because the Dagster chart injects
+  `DAGSTER_HOME=/tmp/dagster` ahead of it.
+- `loosely_coupled_k8s_job` reached `RUN_SUCCESS` on sandbox-cat-dat through the
+  standalone probe (run `cff9b348-bfc3-4ac1-ab51-a94892b8e3a0`), with four
+  external payload pods reporting over the pod-log message channel.
+- Tightly coupled preflight evidence now checks the chart's actual
+  `DAGSTER_PG_PASSWORD` injection without exposing its value.
+- Dagster UI screenshots cover the registered jobs, graph, code-configured
+  execution-target tags and a successful reference run.
+- The platform deployment diagram includes the service and both integration
+  patterns.
+
 ### pending
 
-- Publish the payload image to the container registry.
-- End-to-end run of `loosely_coupled_k8s_job` against a cluster, and confirmation
-  of the Kubernetes rows in the readiness checklist.
-- Screenshots of UI surfaces and integration-pattern diagrams.
-- Platform architecture document update.
+- Extend the project-level GitLab pipeline to publish the payload image as well
+  as the code-location image.