feat(alerts): add scene-local recent ticker

Model the AlertBox and standalone ticker with shared settings; scene tickers render the in-memory active and queued events without saving history. Keep AlertBox content active-event-only.

Ref: https://obsproject.com/kb/obs-studio-overview

HANDOFF.md

@@ -1,31 +1,27 @@
-# HANDOFF — 2026-11-02 (Steam ownership check)
+# HANDOFF — 2026-11-02 (scene-local alert ticker)
 
 ## State
 
-- `main`; local work unit, not pushed.
-- Distribution plan: one paid Steam app ($79 announced launch price, then $99) plus its associated
-  free Demo. Same code; no subscription, DLC purchase, or GitHub binary.
-- `IEntitlement.IsAppOwned` feeds the app-wide `MainViewModel.IsAppOwned` flag. Debug uses a fake
-  owned result; the fake is excluded from Release.
-- Release ownership uses `BIsSubscribedApp(paidAppId)` and fails closed when the ID or Steam is
-  unavailable. **The paid app ID is not configured yet; the Steamworks application is pending.**
-- Steam's private-use clause is not, by itself, a proven blocker. EULA wording should expressly
-  allow commercial/monetized streaming; OBS is the relevant Steam software precedent. See
-  `MyMistakes.md` → Steam distribution.
+- `main`; ticker work is local and not pushed.
+- Standalone `SourceType.Ticker` is addable once per scene, has per-source settings, and shows the
+  in-memory current + queued alert feed. It does not persist event history.
+- Stream Alerts keeps one box per layout; its ticker shares `Ticker` settings but shows only the
+  active event. Existing per-Source columns persist the settings without a schema migration.
+- Focused build succeeded; the new integration test and all `AlertTickerPreviewDisplayTests` pass.
 
 ## Next action
 
-Day 1 next: system-message strip. The queued Debug-only test-access rule also needs checking. Keep
-one integration test per change.
+Next queued unit: Day 1 system-message strip; also check the queued Debug-only test-access rule.
+Keep one integration test per change. Do not push absent a milestone/checkpoint request.
 
 ## Landmines
 
-- The Demo has its own App ID. Ownership must check the paid app ID, never the Demo ID or test ID
-  `480`.
-- No fake ownership code or developer panel in Release.
-- No Steam checkout in the app; Steam handles purchase and updates.
-- The paid app ID is not available yet; Release stays fail-closed until it is configured.
-- `verify.sh` clean app build passes; `EntitlementTests` passes 5/5. Full suite currently has 13–14
-  UI/content failures; connection to this change is unconfirmed, investigate separately.
-- Bundled FFmpeg must stay outside the single-file exe (`ExcludeFromSingleFile="true"`).
-- Build with Windows `dotnet.exe`; run tests per class (the whole suite can hang in WASAPI teardown).
+- Build/test with Windows `dotnet.exe`; the full suite can hang in WASAPI teardown.
+- Focused ticker tests pass; clean app build is 0 warnings. Full suite reports 13–14 UI/content
+  failures (508/521 or 507/521 pass), matching the prior baseline note; none is a ticker test. Their
+  relationship to this change is unconfirmed, and `verify.sh` currently only allows two other tests.
+- The integration assertion is in `ytLive.Tests/AlertTickerPreviewDisplayTests.cs` and must use
+  `LayoutPathOverride`/clear it in `finally` if it constructs `MainWindow`.
+- Scene tickers are dynamic per-element sources; only the AlertBox ticker uses the pump's global
+  ticker overlay callback.
+- Bundled FFmpeg stays outside the single-file exe (`ExcludeFromSingleFile="true"`).