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/OFFSCREEN_RENDERING.md

@@ -336,6 +336,9 @@ to rebuild it twice, 1.7-3.8 ms each on a Pi 4). The `extend` row of
    work" table), and extensions no longer rebuilding the strip's PIL copy.
 3. **Live elements:** the plugin API, the records, the worker and in-place
    patches, with the sports scoreboards and the flight map adopting it.
+4. **Live games in the ticker by default:** `live_in_ticker` true, so a live
+   game's cards update in the marquee instead of the full-screen scoreboard
+   replacing it; existing configs are switched once (`ConfigManager`).
 
 `display.vegas_scroll.offscreen_prefetch` (default `true`) restores the
 deferred path when `false`, and `display.vegas_scroll.live_refresh` (default