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>

test/_run_loop_harness.py

@@ -724,11 +724,14 @@ class RunLoopHarness:
         self.clock.at(t, post)
 
     def restore_on_demand(self, plugin_id: str, mode: Optional[str] = None,
-                          duration: Optional[float] = None, pinned: bool = False):
+                          duration: Optional[float] = None, pinned: bool = False,
+                          named_mode: Optional[str] = None):
         """Start with an on-demand session resumed from the cache, as after
         a restart: the state _select_startup_plugins restores, then
-        _populate_on_demand_modes_from_plugin, as __init__ calls it."""
+        _populate_on_demand_modes_from_plugin, as __init__ calls it. A
+        session that cannot resume is logged as ``on-demand-error``."""
         dc = self.controller
+        dc._on_demand_named_mode = named_mode
         dc.on_demand_active = True
         dc.on_demand_plugin_id = plugin_id
         dc.on_demand_mode = mode
@@ -739,6 +742,8 @@ class RunLoopHarness:
         dc.on_demand_status = 'active'
         dc.on_demand_schedule_override = True
         dc._populate_on_demand_modes_from_plugin()
+        if dc.on_demand_status == 'error':
+            self.log("on-demand-error", dc.on_demand_last_error)
 
     def wifi_message(self, t: float, message: str, duration: float = 5):
         def write():