fix(on-demand): show a named live mode; end a session that cannot resume

A request naming a *_live mode (football-scoreboard / ncaa_fb_live with
15 college games on) answered 200 and showed nfl_recent. The session's
mode list kept live modes only when has_live_content() said so, which is
the live-priority question and is answered for favourite teams only. A
mode the request names now leads the session; display() decides whether
it has anything to draw, and an empty one moves on to the plugin's next
mode as any empty on-demand mode does. The name is saved in
display_on_demand_config (named_mode) so a restart resumes on it. A bare
plugin-id request still skips quiet live modes, as before.

A restart during a session whose plugin then failed to load (clock-simple
failed config validation after a crash on ledpi) left the session active
with no modes and its cached request in place. It now ends at startup
with status error / restore-failed, and the cached request is dropped;
likewise when the plugin system fails to start.

Golden traces: two new scenarios (on_demand_named_live,
on_demand_restore_failed); every existing trace is unchanged. The
harness's restore_on_demand takes named_mode and logs a failed restore.

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

docs/RUN_LOOP_REDESIGN.md

@@ -181,13 +181,14 @@ that the harness patches in today.
   - dynamic duration (cycle complete, plugin cap, global cap)
   - live priority taking over and handing back; live round-robin
   - on-demand start/stop/expiry; pinned on-demand; a session resumed after
-    a restart
+    a restart, and one that cannot resume (its plugin did not load); a
+    request naming a live mode the plugin's live check would drop
   - schedule off and dim, with an on-demand override during downtime
   - WiFi notice; sync follower
   - Vegas, with and without `live_in_ticker`
 - Each trace row is `[start, mode, duration, exit_reason, frames,
   force_clear]`. The exit reason is the event that decided what came next.
-- All 16 tests run in under a second. The goldens were generated from
+- All 18 tests run in under a second. The goldens were generated from
   main's `run()` before any code moved.
 - Vegas uses `FakeVegas`, which implements only the contract the controller
   depends on: `run_iteration()` returns True after its duration and False