[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

README.md

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+# Distributed Execution
+
+Canonical location for documentation, example workflows and reference service
+implementations covering **distributed execution patterns** in the Simpl
+orchestration platform.
+
+The service answers one question for every workflow: *where does the compute
+actually run, and how does runtime information get back to the control plane?*
+
+## Contents
+
+| Area | Location | Status |
+|---|---|---|
+| User guide | [documents/user-guide/distributed-execution-guide.md](documents/user-guide/distributed-execution-guide.md) | Complete |
+| Readiness checklist | [documents/user-guide/readiness-checklist.md](documents/user-guide/readiness-checklist.md) | Complete |
+| Tightly coupled reference | [src/distributed_execution/tightly_coupled/jobs.py](src/distributed_execution/tightly_coupled/jobs.py) | Runnable |
+| Loosely coupled reference | [src/distributed_execution/loosely_coupled/jobs.py](src/distributed_execution/loosely_coupled/jobs.py) | Runnable (subprocess verified) |
+| External payload | [payload/work.py](payload/work.py) | Runnable |
+| Example configuration | [yaml/](yaml/) | Complete |
+
+## Project structure
+
+```text
+distributed-execution/
+├── src/
+│   └── distributed_execution/
+│       ├── repository.py           # Dagster definitions (entry point)
+│       ├── ops.py                  # Shared ops used by both patterns
+│       ├── preflight.py            # Readiness checks backing the checklist
+│       ├── tightly_coupled/
+│       │   └── jobs.py             # in_process / multiprocess / k8s_job_executor
+│       └── loosely_coupled/
+│           └── jobs.py             # PipesSubprocessClient / PipesK8sClient
+├── payload/                        # External workload: dagster-pipes ONLY
+│   ├── work.py
+│   ├── requirements.txt
+│   └── Dockerfile
+├── documents/user-guide/           # AC1-AC4 documentation
+├── yaml/                           # Working example configuration
+├── tests/
+├── Dockerfile
+├── pyproject.toml
+└── workspace.yaml
+```
+
+## Getting started
+
+Prerequisites: Python 3.12+ and `uv`.
+
+```bash
+uv sync --dev
+uv run dagster dev -f src/distributed_execution/repository.py
+```
+
+The Dagster UI is then available at <http://localhost:3000>. Two jobs run end to
+end on a laptop with no cluster: `tightly_coupled_local_job` and
+`loosely_coupled_subprocess_job`. The Kubernetes variants of each are documented
+in the user guide.
+
+### Running tests
+
+```bash
+uv run pytest
+```
+
+### Building the images
+
+Two images, deliberately: the code location and the external payload are
+versioned and scanned independently.
+
+```bash
+docker build -t distributed-execution:0.1.0 .
+docker build -f payload/Dockerfile -t distributed-execution-payload:0.1.0 payload/
+```
+
+## Status
+
+Both execution targets are implemented. The tightly coupled jobs and the loosely
+coupled **subprocess** transport are verified by the test suite. The loosely
+coupled **Kubernetes** transport is implemented but has not yet been run against
+a cluster; see the guide's *Outstanding work* section.
+
+## Licence
+
+European Union Public Licence v1.2 — see [LICENSE](LICENSE).