Working / idle / hung indicator for one harness turn.

since_ms is the elapsed time, in milliseconds, that the turn has spent in its current state -- not a wall-clock timestamp. That keeps render/2 a pure function of props (no System.monotonic_time/1 inside render, so the component stays trivially testable); the caller's own tick/subscription loop accumulates it, the same way Raxol.Playground.Demos.StatusBarDemo accumulates model.tick.

When state: :working has run longer than hung_threshold_ms, the indicator flips itself to the hung display even though the caller still claims :working -- that override is the point: it catches the agent that died but still looks like it is working.