Needs to be frame rate independent.

Right now our movement and updates are at the mercy of the frame refresh rate. Need to put in some explicit reading of the clock and measuring elapsed time to be independent of frame rate. See the discussion in this hackernoon link: https://hackernoon.com/how-to-build-a-multiplayer-browser-game-4a793818c29b

Start by locating the game loop and the movement and update entry points in the JavaScript implementation. Read the linked HackerNoon discussion for the elapsed-time approach, then trace where frame refresh rate currently affects updates. Done means movement and updates behave consistently across different frame rates, with the existing game behavior preserved.

Written by the indexing model from the issue text.