feat(FN-2950): merge fusion/fn-2950

- Updated routing policy documentation in `docs/architecture.md` and `docs/settings-reference.md`
- Revised docs reflect the latest routing configuration options and behavior

Commits merged:
- feat(FN-2950): complete Step 6 — update routing policy documentation

Files changed:
docs/architecture.md       | 19 ++++++++++---------
 docs/settings-reference.md |  2 +-
 2 files changed, 11 insertions(+), 10 deletions(-)

Fusion-Task-Id: FN-2950

packages/engine/src/node-routing-policy.ts

@@ -0,0 +1,45 @@
+import type { NodeStatus, UnavailableNodePolicy } from "@fusion/core";
+
+export interface PolicyResult {
+  allowed: boolean;
+  fallbackToLocal: boolean;
+  reason: string;
+}
+
+const UNHEALTHY_STATUSES: ReadonlySet<NodeStatus> = new Set(["offline", "error", "connecting"]);
+
+export function applyUnavailableNodePolicy(
+  nodeStatus: NodeStatus | undefined,
+  policy: UnavailableNodePolicy | undefined,
+  isLocal: boolean,
+): PolicyResult {
+  if (isLocal) {
+    return { allowed: true, fallbackToLocal: false, reason: "local-execution" };
+  }
+
+  if (nodeStatus === undefined) {
+    return { allowed: true, fallbackToLocal: false, reason: "unknown-health" };
+  }
+
+  if (nodeStatus === "online") {
+    return { allowed: true, fallbackToLocal: false, reason: "healthy" };
+  }
+
+  if (!UNHEALTHY_STATUSES.has(nodeStatus)) {
+    return { allowed: true, fallbackToLocal: false, reason: "healthy" };
+  }
+
+  if (policy === "fallback-local") {
+    return {
+      allowed: true,
+      fallbackToLocal: true,
+      reason: `fallback-local:${nodeStatus}`,
+    };
+  }
+
+  return {
+    allowed: false,
+    fallbackToLocal: false,
+    reason: `blocked:${nodeStatus}`,
+  };
+}