[Full-Stack Signals] implement latency compensation

[cromoteca]

Describe your motivation

Signal commands should be applied immediately on the client, then reverted if failed on the server.

Depends on #3967.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

[platosha]

Barebones structure of the implementation in terms of signals could look like:

const confirmedValue = signal(initialValue);

function commandInterpreter(prevValue, command: Command) {
  // implement commands here
}

const unconfirmedCommands = signal<readonly Command[]>([]);
const unconfirmedCommandHanlders = new Map<string, {
  readonly resolve: (value) => void,
  readonly reject: (reason) => void,
}>();

function addUserCommand(command: Command) {
  // Add the command to the unconfirmed list and its callbacks to the map
  const { promise, resolve, reject } = Promise.withResolvers();
  unconfirmedCommandHanlders.set(command.id, { resolve, reject });
  unconfirmedCommands.value = [...pendingCommands, command];
  // Notify the server
  signalEndpoint(userValue.value, command);
  // Return the promise to the user
  return promise;
}

signalFlux.subscribe((confirmedCommand: Command) => {
  batch(() => {  
    // Interpret command regardless of where it's coming from and update the `confirmedValue`
    const [ value, commandResult ] = commandInterpreter(confirmedValue.value, confirmedCommand);
    confirmedValue.value = value;
    if (unconfirmedCommandHandlers.has(confirmedCommand.id)) {
      // The command is coming from this client
      unconfirmedCommands.value = pendingCommands
        .filter((command) => command.id !== confirmedCommand.id);
      const { resolve, reject } = unconfirmedCommandHandlers.get(confirmedCommand.id);
      unconfirmedCommandHandlers.delete(confirmedCommand.id);
      if (commandResult.ok) {
        resolve(commandResult.value);
      } else {
        reject(commandResult.reason);
      }
    } else {
      // Someone else's command, nothing else to do
    }
  });
});

const userValue = computed(() => {
  const [ value, ] = unconfirmedCommands.reduce(commandInterpreter, confirmedValue.value);
  return value;
});