Fix README protocol diagram rendering

[Copilot]

The README’s protocol diagram failed to render (Mermaid parse error) when showing the client/server message flow.

• 🛠️ Documentation: Replaced the invalid Mermaid flowchart with a valid sequence diagram that depicts length send/ack, payload transfer, counting, and printable-count response.
• 📄 Clarity: Maintained the same protocol steps while using Mermaid syntax GitHub renders reliably.

sequenceDiagram
    participant Client
    participant Server
    Client->>Server: N (uint32, network byte order)
    Server-->>Client: Length accepted
    Client->>Server: N bytes payload
    Server->>Server: Count printable bytes
    Server-->>Client: C (uint32, network byte order)

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