… routing

…p grace is reserved

Adds a top-level trigger param (interval|on_demand) to RTStream.understand()
and documents the cua (computer-use) analyzer with past_window. The cua
analyzer and past_window already flow through the analyzers list.

…nd CUA loop

understanding.next(step_id) → POST /rtstream/{id}/understand/{und}/next. The per-action
poke of the on-demand (trigger=on_demand) computer-use loop: after the client executes an
action and the screen settles, next() has the model analyze the freshest frame and emit
the next action. understanding_id is supplied automatically (self.id); the worker is
idempotent on the monotonic step_id, so a duplicate/late next is safely ignored. No-op on
an interval understanding. + ApiPath.next.

Part of the on-demand CUA chain (ENG-1719); Server route rtstream-cua-next / #980.