[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

payload/Dockerfile

@@ -0,0 +1,30 @@
+# External payload image for the loosely coupled execution target.
+#
+# Built and versioned independently of the code location image. It contains no
+# workflow code and no `dagster` package - only the pipes protocol client.
+#
+#   docker build -f payload/Dockerfile -t distributed-execution-payload:0.1.0 payload/
+
+FROM python:3.12-slim-bookworm
+
+RUN python -m pip install --no-cache-dir --upgrade "pip==26.1.2"
+
+WORKDIR /app
+
+RUN addgroup --gid 1000 appgroup && \
+    adduser --uid 1000 --gid 1000 --disabled-password --gecos "" appuser
+
+RUN apt-get update && apt-get upgrade -y \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY work.py .
+
+RUN chown -R appuser:appgroup /app
+
+USER appuser
+
+CMD ["python", "/app/work.py"]