test(xtream): pin the series page's forced-launch bookkeeping

Cover the three behaviours of PR #1792's forced MPV/VLC launch that had no
spec: a regular start made while a forced launch is pending is queued and
starts once the launch settled; the launch-position marker is skipped when
the page token changed before the launch settled, or while the position
saved; and the menu reports a failed launch only to the visit that asked
for it, while `startPending` holds the watched actions and the external row.

Each test fails when its branch is removed. The playback spec releases any
launch a failed test left open, since the launch chain is module state.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/architecture/xtream-portal-compatibility.md

@@ -440,5 +440,7 @@ The series page keeps its launch state at module level in
 
 Regression coverage: `serial-details-external-launch.spec.ts` (chain,
 duplicate guard, queued choice), `serial-details-playback.service.spec.ts`
-(page token) and, for the external-player and reset rows,
+(page token, the queued start, the launch-position marker),
+`serial-details-menu.service.spec.ts` (launch-failure message, the rows held
+while a launch is pending) and, for the external-player and reset rows,
 `libs/ui/components/src/lib/detail-ui/series-hero.state.spec.ts`.