feat(streaming): suppress WS echo via origin tagging

Inject _origin=ws:<clientId> into JSON payloads on inbound WS messages and strip it on broadcast while skipping the origin client. Documents behavior in StreamingAPI.md.

docs/StreamingAPI.md

@@ -35,6 +35,15 @@ Notes:
 - The device accepts `payload` as a string or a JSON object/array. Objects are serialized into a string before dispatching to local subscribers to keep a consistent downstream contract.
 - A minimal ack `{ "ok": true }` is sent after a valid inbound message.
 
+#### Echo suppression (origin tagging)
+
+- To prevent the sender from receiving an immediate echo of its own message, the server injects a private field into JSON payloads:
+  - `_origin: "ws:<clientId>"`
+- When re-broadcasting local events to WebSocket clients, the server:
+  - Strips the `_origin` field from the outgoing payload
+  - Skips the originating `clientId` so only other clients receive the message
+  - If a payload is not valid JSON (plain string), no origin tag is injected and the message may be echoed
+
 ### Event Bus Integration
 
 - The WebSocket registers an `onAny` subscriber to `NodeContext` so that all local events are mirrored to clients.