docs: strengthen production network discovery engineering contract

[masarray]

Summary

Upgrades the existing lightweight contributor notes into a production engineering contract tailored to ARNet Discovery's WPF/Core/CLI architecture and real substation-LAN usage.

Key protections

• root-cause-first workflow with a three-patch circuit breaker
• explicit network-safety boundary: bounded targets, concurrency, timeouts and retries
• evidence-oriented classification: ports/DNS/OUI/ARP remain hints, never definitive device identity
• expected reachability/probe failures use Try/typed Result/status semantics instead of exception-driven control flow
• worker callbacks must not assume WPF dispatcher affinity
• batched/coalesced UI publication and virtualized large tables
• bounded asynchronous diagnostics with aggregation/rate limiting
• coherent snapshot/staged-enrichment semantics and deterministic cancellation lifecycle
• measured scan throughput, time-to-first-device, UI latency, allocation/GC and cancellation expectations
• existing Release build/test/package verification CI remains authoritative

Documentation-only change; runtime behavior is unchanged.