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>

CHANGELOG.md

@@ -488,6 +488,26 @@ policies are unchanged.
 
 ### Fixes
 
+- An on-demand request that names a `*_live` mode now shows that mode. On
+  ledpi, `{"plugin_id": "football-scoreboard", "mode": "ncaa_fb_live"}` with
+  15 college games on answered 200 and showed `nfl_recent`. The session's
+  mode list kept a live mode only when the plugin's `has_live_content()`
+  said so. That method answers the live-priority question, and the sports
+  plugins answer it for favourite teams only. A mode the request names
+  (not one resolved from a bare plugin id) now leads the session, with the
+  plugin's other modes after it. If it has nothing to draw, the session
+  moves on to the next of those modes, like any empty on-demand mode. The
+  name is saved with the session (`named_mode` in
+  `display_on_demand_config`), so a restart resumes on it.
+- A restart during an on-demand session whose plugin then fails to load no
+  longer leaves a session with no modes. On ledpi, `clock-simple` failed
+  config validation after a crash. The display logged `No valid display
+  modes found for on-demand plugin 'clock-simple' after restoration` and
+  kept reporting the session as active until its first pass ended it as
+  `idle`. The cached request stayed behind for the next restart. The session
+  now ends at startup with status `error` and error `restore-failed`, which
+  `/display/on-demand/status` reports, and the cached request is dropped. The
+  same applies when the plugin system itself fails to start.
 - The garbage-collection timer (`GcMonitor`, above) no longer prints
   `Exception ignored while calling GC callback ... 'NoneType' object has no
   attribute 'perf_counter'` when the display service or a test run exits.