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>

test/test_ipc_display_hook.py

@@ -162,6 +162,14 @@ class TestLifecycle:
         assert status['on_demand']['plugin_id'] == 'clock'
         c.encode_message(status)   # it has to fit on the wire
 
+    def test_the_state_names_the_request_it_answers(self, controller):
+        # The web interface keeps reporting a delivered start as "starting"
+        # until the display publishes state for that request id.
+        assert controller._on_demand_state()['request_id'] is None
+        controller._control_server = FakeServer(_start('sock-7'))
+        controller._poll_on_demand_requests()
+        assert controller._on_demand_state()['request_id'] == 'sock-7'
+
     def test_cleanup_closes_the_socket(self, controller):
         server = FakeServer()
         controller._control_server = server