End-to-end multi-node cluster: Phase 1 LAN testnet

## Description

The whitepaper's staged release plan requires Phase 1: "3–5 Machine LAN Testnet" on physical machines (not nested VMs). This validates the core cluster formation, job execution, preemption, and failure recovery in a real network environment.

## Requirements (from whitepaper Phase 1)

- Physical machines — not nested VMs — on a real network
- At least one ARM machine
- Peer discovery succeeds without manual IP configuration (mDNS within 2 seconds)
- A node failure mid-job is detected by missed heartbeat; job reschedules from checkpoint
- Resource yield occurs within 1 second of simulated keyboard event
- No cross-node data leakage
- Kill conditions: any cross-node sandbox breach, host OOM, or data loss from simulated node failure

## Success Criteria

- [ ] 3+ physical machines form cluster via mDNS in < 5 seconds
- [ ] R=3 job executes correctly across nodes
- [ ] Node failure → job rescheduled from checkpoint → correct result
- [ ] Preemption within 1 second of keyboard activity
- [ ] Zero cross-node data leakage
- [ ] No host residue after job completion
- [ ] At least one ARM node participating
- [ ] Evidence artifact published with all test results

## Testing (Principle V)

This IS the test — it must run on real hardware:
- 3-5 diverse physical machines (x86 + ARM)
- Real LAN (not virtual network)
- Real workloads (not toy assertions)
- Deliberate failure injection (kill node, disconnect cable)
- Keyboard event → measure preemption latency
- Inspect each host after tests for residual files/processes