Function foldRenderWindow

foldRenderWindow(
    stored: RenderWindow | undefined,
    pending: number,
    nowMs: number,
    windowMs?: number,
    survived?: boolean,
    pendingColds?: number,
    ceilingMs?: number,
    lastAt?: number,
): RenderWindow
Folds pending renders into the stored window, rolling it over when it has aged out.

Rolls rather than accumulates, so a site that was busy last month cannot keep itself warm on it.
Parameters
- stored: RenderWindow | undefined
- pending: number
- nowMs: number
- windowMs: number = RATE_WINDOW_MS
- survived: boolean = false
  whether ONE incarnation spanned the window that is closing; see windowVerdict
- pendingColds: number = 0
- ceilingMs: number = DEFAULT_CEILING_MS
- OptionallastAt: number
  when the newest of the pending renders arrived; kept from the stored window when absent
Returns RenderWindow
- Defined in src/ops/thermal.ts:272