docs(cluster-infrastructure): resolve ClusterInfrastructure-001 — document that the Host owns the Akka bootstrap

docs/requirements/Component-ClusterInfrastructure.md

@@ -18,6 +18,26 @@ Both central and site clusters.
 - Support cluster singleton hosting (used by the Site Runtime Deployment Manager singleton on site clusters).
 - Manage Windows service lifecycle (start, stop, restart) on each node.
 
+## Implementation Note — Code Placement
+
+This component is a **design responsibility**, not a single buildable project that
+contains all of the code. The cluster-infrastructure responsibilities above are
+realised across two projects:
+
+- **`src/ScadaLink.ClusterInfrastructure`** owns the cluster **configuration model**:
+  the `ClusterOptions` POCO (seed nodes, roles, remoting/gRPC ports, failure-detection
+  timings, split-brain settings) bound from `appsettings.json` via the Options pattern.
+- **`src/ScadaLink.Host`** owns the cluster **bootstrap and runtime wiring**: it
+  builds the Akka.NET HOCON from `ClusterOptions`, starts the `ActorSystem`,
+  configures the keep-oldest split-brain resolver (`down-if-alone = on`), wires
+  `CoordinatedShutdown` into the service lifecycle, and provides active-node /
+  cluster-membership health checks. See `Component-Host.md` (REQ-HOST-*) for detail.
+
+This split is deliberate — the Host is the single deployable binary and the only
+project that performs Akka.NET bootstrap, so the cluster bring-up lives there
+alongside role-based component registration. The `ClusterInfrastructure` project
+remains the home of the configuration contract that the Host consumes.
+
 ## Cluster Topology
 
 ### Central Cluster