fix(web): a delivered on-demand start reads as starting until the display acts on it

On ledpi (three cold starts) the display acknowledged the start as its
socket opened, then took ~5 s to act on it while Vegas built its first
strip; the status routes meanwhile showed the display's own idle state, so
a UI polling every 700 ms flashed idle.

The display's on-demand state now names the request it answers
(request_id). A delivered start keeps reading as status "starting" with
delivered: true, in /display/on-demand/status and as on_demand_pending in
/display/current-status, until the display publishes state for that
request id (a display without the field: any state newer than the
delivery), for at most DELIVERED_SHOWN_SECONDS (30 s). The display's
startup state, which can be published after the acknowledgement, names no
request and does not end it.

Tests: stays starting against the startup idle state (no id, an older id);
the matching active state and the matching error take over; an older
display's newer state takes over; the 30 s cap; the display's state names
its request. Mutation check: 11 mutants, 11 killed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/display_controller.py

@@ -1791,6 +1791,9 @@ class DisplayController:
             'status': self.on_demand_status,
             'error': self.on_demand_last_error,
             'last_event': self.on_demand_last_event,
+            # The request this state answers: lets the web interface tell
+            # the outcome of a start it delivered from an older state.
+            'request_id': self.on_demand_request_id,
             'remaining': self._get_on_demand_remaining(),
             'last_updated': time.time()
         }