Headless: add a readiness signal and let cui png --wait use it #6

Context

Any app whose first frames are empty must block startup so cui png / CUISHARP_HEADLESS captures real content. In my dashboard:

if (Environment.GetEnvironmentVariable("CUISHARP_HEADLESS") is "1" or "true")
    tracker.WaitForFirst(TimeSpan.FromSeconds(25));   // otherwise the PNG is an empty shell

That arbitrary sleep is wall-clock tax on every screenshot and CI run, and every data-driven app will reinvent it.

Proposal

• A first-class signal: ui.MarkReady() on the frame that may be considered settled, or CuiAppOptions.Ready = () => bool.
• HeadlessLoop records it. cui png --wait ready blocks until it appears (with a timeout), and --wait-text "Updated 12:34" covers apps that do not opt in.
• Keep --frames for the existing behaviour.

Acceptance

cui png out.png --wait ready returns as soon as live data lands, with no app-level sleep, and fails with a clear timeout message otherwise.