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>

CHANGELOG.md

@@ -406,6 +406,15 @@ read any of them:
   `render_vegas_strip()` and `render_vegas_timeline()` in
   `src/plugin_system/testing/vegas.py`; the join is now
   `render_pipeline.join_plugin_rows()`.
+- **Behaviour change: live games stay in the Vegas ticker by default.**
+  `display.vegas_scroll.live_in_ticker` now defaults to `true`: the marquee
+  keeps running through a live game, which takes extra turns in it, instead
+  of giving way to the full-screen scoreboard. Existing configs all held the
+  old `false`, copied from the template, so the first start turns it on once
+  (`ConfigManager._migrate_live_in_ticker_default`; the previous config is
+  kept as `config.json.backup` and `live_in_ticker_migrated` records that it
+  ran). To keep the full-screen scoreboard, untick the new **Keep live games
+  in the ticker** under Vegas mode; a `false` set after the migration stays.
 
 ### Scrolling