Add per-client slot disconnect reason to the server

[gafferongames]

Summary

The connect_disconnect_callback only reports (client_index, connected), so server code cannot tell why a client disconnected — did it leave cleanly (disconnect packet), time out, or get disconnected by server code? This is the key "is it us or them" signal for game backends built on netcode (motivated by mas-bandwidth/yojimbo#290, which exposes disconnect reasons at the yojimbo layer but could not distinguish timeout from clean disconnect without this).

• New function int netcode_server_client_disconnect_reason( server, client_index ) returning:
  • NETCODE_SERVER_CLIENT_DISCONNECT_REASON_NONE — no disconnect in this slot yet (or a new client has since connected to it)
  • NETCODE_SERVER_CLIENT_DISCONNECT_REASON_TIMED_OUT — client went silent past its timeout
  • NETCODE_SERVER_CLIENT_DISCONNECT_REASON_CLIENT_DISCONNECT — received a disconnect packet from the client (clean)
  • NETCODE_SERVER_CLIENT_DISCONNECT_REASON_SERVER_DISCONNECT — server code called netcode_server_disconnect_client / disconnect_all_clients / disconnect_loopback_client
• Tracked per-client slot: reset to NONE at netcode_server_start and when a new client connects to the slot. Recorded before connect_disconnect_callback fires, so it can be queried from inside the callback.
• Existing functions unchanged; the internal disconnect function gains a reason parameter, set at each call site where the cause is known.

Test plan

• New test_server_client_disconnect_reason: reason is NONE at server start and while connected → client goes silent → server times it out → TIMED_OUT → reconnect clears the slot back to NONE
• test_client_side_disconnect now asserts CLIENT_DISCONNECT and test_server_side_disconnect asserts SERVER_DISCONNECT — directly verifying their stated intent ("disconnects cleanly, rather than timing out")
• Full suite passes in Debug and Release, builds warning-free
• CI matrix (Linux x64/arm64, macOS, Windows MSVC + MinGW, ASan+UBSan, smoke fuzz)

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