[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

src/distributed_execution/preflight.py

@@ -0,0 +1,61 @@
+"""Readiness checks backing the pre-run validation checklist.
+
+Each function returns a plain dict so the same result can be logged as Dagster
+metadata (evidence for the checklist) or asserted in a test.
+"""
+
+from __future__ import annotations
+
+import os
+import socket
+from urllib.parse import urlparse
+
+# Runtime dependencies a tightly coupled execution pod must be able to resolve.
+TIGHTLY_COUPLED_ENV_VARS = (
+    "DAGSTER_POSTGRES_HOST",
+    "DAGSTER_POSTGRES_USER",
+    "DAGSTER_POSTGRES_DB",
+)
+
+
+def check_env_vars(names: tuple[str, ...] | list[str]) -> dict:
+    """Report which of ``names`` are present, without echoing their values."""
+    present = [name for name in names if os.environ.get(name)]
+    missing = [name for name in names if not os.environ.get(name)]
+    return {
+        "check": "env_vars",
+        "passed": not missing,
+        "present": present,
+        "missing": missing,
+    }
+
+
+def check_tcp_reachable(host: str, port: int, timeout: float = 3.0) -> dict:
+    """Open a TCP connection to prove the execution pod can reach a dependency."""
+    result = {"check": "tcp_reachable", "target": f"{host}:{port}", "passed": False, "error": None}
+    try:
+        with socket.create_connection((host, port), timeout=timeout):
+            result["passed"] = True
+    except OSError as exc:
+        result["error"] = str(exc)
+    return result
+
+
+def check_endpoint_reachable(url: str, timeout: float = 3.0) -> dict:
+    """TCP-level reachability for an endpoint expressed as a URL."""
+    parsed = urlparse(url)
+    if not parsed.hostname:
+        return {"check": "tcp_reachable", "target": url, "passed": False, "error": "no hostname in URL"}
+    port = parsed.port or (443 if parsed.scheme == "https" else 80)
+    return check_tcp_reachable(parsed.hostname, port, timeout=timeout)
+
+
+def describe_pod_identity() -> dict:
+    """Where this process is actually running - the primary execution-target evidence."""
+    return {
+        "hostname": socket.gethostname(),
+        "pid": os.getpid(),
+        "namespace": os.environ.get("DAGSTER_K8S_PIPELINE_RUN_NAMESPACE", "<not-in-kubernetes>"),
+        "run_id": os.environ.get("DAGSTER_RUN_ID", "<unset>"),
+        "image": os.environ.get("DAGSTER_K8S_PIPELINE_RUN_IMAGE", "<unset>"),
+    }