docs: add delta-v telemetryd architecture document

[pbrane]

Summary

Adds docs/architecture/deltav-telemetryd-architecture.md documenting the telemetry ingestion architecture:

• The three-tier pipeline: Minion (UDP listener + 4-byte protocol sniff) → minion-gateway (per-protocol gRPC → per-protocol Kafka topics) → flow-enricher (SCS consumer → ClickHouse), with telemetryd as a vestigial/extension-seat bridge (all adapters disabled).
• Protocol-to-protocol coupling analysis: per-protocol isolation end-to-end (topics, gRPC methods, sink modules, processors) vs. the three shared chokepoints (single UDP port, shared gateway plumbing, deploy-time config).
• Minion coupling: the two thin wire contracts (TelemetryService gRPC proto, DeltaV.Sink.* + SinkMessage framing) and the content-blind design.
• Horizontal scaling via Spring Cloud Stream: flow-enricher and prometheus-writer as SCS consumers (consumer-group scale-out), the SCS producers feeding them (collectd, pollerd/perspectivepollerd via poller-timeseries-common, provisiond), and the scaling ceilings (4 partitions per topic, <location>@<minion-id> key skew, raw kafka-clients services that scale manually).
• Mermaid diagram of the full pipeline.

Documentation only — no code changes.

https://claude.ai/code/session_01E1cRKma4wRcjD6FmtVriA3