feat(vegas): keep live games in the ticker by default

display.vegas_scroll.live_in_ticker now defaults to true: through a live
game the marquee keeps running and the live scoreboard takes extra turns in
it -- its cards updating in place while they scroll -- instead of the ticker
giving way to the full-screen scoreboard.

The new default would reach nobody on its own: every existing config holds
an explicit false copied from the template (there was no control for it),
and the template merge only adds missing keys. ConfigManager therefore turns
a stored false on once, with a backup, and records live_in_ticker_migrated
so a false chosen afterwards stays. The marker is never in the template.

A "Keep live games in the ticker" checkbox under Vegas mode sets it. Tests
that pin the full-screen takeover now say live_in_ticker=false.

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

docs/PLUGIN_API_REFERENCE.md

@@ -308,9 +308,9 @@ the core then falls back to its own live-content check — so a plugin whose
 weight calculation is broken still gets `live_weight` for a game that really
 is live, rather than being demoted to 1.
 
-Only consulted when the user has set `vegas_scroll.live_in_ticker`. With the
-default (`false`) live content preempts Vegas entirely and there is no ticker
-to be weighted within. See
+Only consulted while `vegas_scroll.live_in_ticker` is on (the default since
+3.8.0). With it off live content preempts Vegas entirely and there is no
+ticker to be weighted within. See
 [ADVANCED_FEATURES.md](ADVANCED_FEATURES.md#live-content-in-the-ticker).
 
 ### Vegas scroll hooks