Add targeted [[nodiscard]] annotations to critical result-returning APIs

[Copilot]

This PR tightens compile-time misuse detection by marking a small set of high-signal return values as [[nodiscard]]. Scope is intentionally narrow: only validity checks and network send/receive outcomes where ignoring results can mask faults.

• What changed

  • Added [[nodiscard]] to critical validators in state/order models:
    • ElevatorState::Valid(), ElevatorState::Usable()
    • Order::Valid(), OrderTable::Valid()
    • Order::ObservedBy(int) (observation-state check)
  • Added [[nodiscard]] to network outcome APIs:
    • UdpBroadcaster::SendPacket(...)
    • UdpReciever::RecievePacket(...)
    • NetworkPacket::Valid()

• Scope boundaries

  • Header declarations only.
  • No behavioral, control-flow, or formatting changes outside these annotations.

• Example

[[nodiscard]] bool Valid() const;
[[nodiscard]] bool SendPacket(NetworkPacket* packet);
[[nodiscard]] bool RecievePacket(NetworkPacket* packet);

• Fixes fix: add nodiscard to functions #7

[eskfro]

Nice.