Add formal Network Layer architecture documentation from code-level analysis

[Copilot]

This update addresses the request for a report-ready Network Layer architecture section grounded strictly in the repository’s implemented behavior. It adds a structured analysis artifact, a section/diagram plan, and a complete architecture document with embedded Mermaid diagrams.

• Scope and analysis artifacts

  • Added _memory.md capturing detailed understanding of network-layer responsibilities, component interactions, data flow, and layer boundaries.
  • Added _plan.md defining the architecture section structure and diagram set, mapped to concrete code areas.

• Architecture report deliverable

  • Added network_layer_architecture.md with full formal content under # Network Layer Architecture.
  • Documented:
    • Layer boundaries and interfaces (NetworkManager, ILinkLayer, ILocationProvider)
    • Frame/header model and message semantics
    • Routing pipeline (TTL, duplicate, hop, distance, directional cone, hold-back)
    • Forwarding strategy (queueing, priority eviction, implicit cancellation)
    • Concurrency/threading model
    • Runtime/configuration constraints
    • Host simulation architecture, configuration parser behavior, and event/channel models

• Diagram coverage (Mermaid, in-file)

  • Included architecture, sequence, flow, lifecycle, threading, composition, and event-pipeline diagrams directly in context-relevant sections.

Example snippet from the new architecture file:

flowchart TD
    A[Receive NetworkHeader + RSSI + SNR] --> B{TTL expired?}
    B -- Yes --> X1[Verdict: DROP]
    B -- No --> C{Duplicate message_id?}
    C -- Yes --> X1
    C -- No --> D{hops_remaining == 0?}
    D -- Yes --> X2[Verdict: DELIVER_ONLY]
    D -- No --> E{max_distance_m enforced and exceeded?}
    E -- Yes --> X2
    E -- No --> F{prop_mode == DIRECTIONAL?}

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