docs(adr): ADR-083 — per-cluster Pi compute hop (proposed)

[ruvnet]

Summary

Adopt one Pi per cluster of 3-6 ESP32-S3 sensor nodes as the canonical RuView mid-scale deployment shape, rather than the full three-tier (dual-MCU + per-node Pi) shape from docs/research/architecture/three-tier-rust-node.md.

This is the L3-hybrid path identified in docs/research/architecture/decision-tree.md §2 — the cheapest viable upgrade. Sensor nodes are unchanged from ADR-028 / ADR-081; the cluster Pi gains the responsibilities the ESP32-S3 cannot carry:

1. Per-cluster ML inference — WiFlow / DT-Pose / GraphPose-Fi class models (4-10M params, 0.5-1.5 GFLOPs) want a Cortex-A53-class target. ESP32-S3 cannot host these; cloud can but only at the cost of round-trip latency.
2. QUIC backhaul — quinn + rustls is mature on Linux but does not run on ESP32-class hardware in any production-grade form (SOTA §5).
3. Cluster-level OTA + secure-boot anchor — buildroot + dm-verity + signed FIT on the Pi as the trust anchor for its sensors' rollouts.

Status

Proposed — not committing to implementation. Acceptance gated on:

1. Cross-compile to aarch64-unknown-linux-gnu / armv7-unknown-linux-gnueabihf with workspace tests passing
2. 3-sensor + 1-Pi field test demonstrating end-to-end CSI → fusion → cloud at ≤100 ms cluster latency
3. Cluster-Pi SoC choice ADR (decision-tree.md L6) approved

What this ADR does NOT decide

• Pi SoC choice (Pi Zero 2W vs CM4 vs Pi 5) — deferred to a future ADR (decision-tree.md L6 / secure-boot threat model).
• no_std CSI maturity (decision-tree.md L4) — not required for the cluster-Pi shape; only required for dual-MCU.
• Per-node Pi shape — explicitly not this ADR. Stays as a future option.

Validation in this PR

• ADR text references existing ADRs correctly (ADR-028, ADR-029, ADR-066, ADR-081, ADR-082).
• All path references already on v2/ (rebased on top of chore(repo): rename rust-port/wifi-densepose-rs → v2/ (flatten to one level) #427's rename merge).
• No code changes — pure ADR document.

Test plan

• ADR text correctly references current main (post-chore(repo): rename rust-port/wifi-densepose-rs → v2/ (flatten to one level) #427 rename)
• Cross-references to research dossier resolve (docs/research/architecture/three-tier-rust-node.md, decision-tree.md, sota/2026-Q2-rf-sensing-and-edge-rust.md)
• Reviewer accepts the L3-hybrid framing as the right starting point

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