feat(protocol): carry stream policy and gateway telemetry

openspec/changes/phase3c-gateway-heartbeat-telemetry/design.md

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+## Context
+
+The Data Plane already collects process-wide atomic counters and gauges. Only active connections and egress Kbps cross the authenticated heartbeat boundary.
+
+## Goals / Non-Goals
+
+**Goals:**
+
+- Carry the existing low-cardinality observations with explicit units.
+- Bound every numeric field and enumerate provider state.
+- Keep registration capacity distinct from measured traffic.
+
+**Non-Goals:**
+
+- Add session, route, endpoint, credential, label, or payload fields.
+- Define a new telemetry transport.
+- Publish a Protocol version.
+
+## Decisions
+
+- Nest the values in required `GatewayTelemetry` so heartbeat telemetry is one strict atomic contract.
+- Use cumulative counters and microsecond delay totals plus processing samples; consumers can derive rates/averages without losing raw observations.
+- Keep loss as parts per million and provider state as a bounded enum.
+
+## Risks / Trade-offs
+
+- [Cumulative counters approach signed integer limits] → Bound at signed 64-bit and saturate consumer conversions.
+- [New required object breaks RC6] → Test locally and publish only under separate immutable-version authorization.
+
+## Migration Plan
+
+Regenerate all bindings locally, update both consumers through the temporary workspace, and stop at the immutable publication boundary.
+
+## Open Questions
+
+None.