Add OTLP observability and SigNoz demo runtime

[jillesvangurp]

Adds first-class OTLP observability to the hub, wires the local demo stack to SigNoz by default, and documents/tests the new telemetry baseline.

Highlights:

• OTLP metrics, traces, and logs with environment-driven runtime config
• ingest, queue, fence, collision, MQTT, WebSocket, auth, metadata, and RPC instrumentation
• local SigNoz-backed demo stack and connector defaults for persistent telemetry inspection
• integration coverage proving OTLP logs, metrics, and traces are emitted end to end
• CI cache wiring for the pinned SigNoz checkout used by demo/e2e flows