test(desktop): make prompt-rail tail mounting deterministic

[liugddx]

Summary

The prompt-rail eviction Desktop e2e can set the transcript to the bottom without causing the turn virtualizer to observe a scroll event. The test then waits for the exact tail turn (turn-prompt-rail-120), but the mounted window remains stale and the assertion times out.

Evidence

An unrelated Windows-storage PR (#3789) failed on exact head b98f3b329 after all build, release-contract, workspace, Eval, Runtime Host, and Windows recovery checks passed:

• Desktop e2e failure
• scenario: evicting a turn-owned sibling interaction hands focus back to the transcript
• expected [data-virtual-turn-id="turn-prompt-rail-120"]: 1
• observed for 10 seconds: 0

#3789 has no Desktop/UI diff against main.

Cause boundary

scrollTranscriptTo only assigns scroller.scrollTop. The virtualized window is updated through the scroll observation path, but a programmatic assignment does not provide a deterministic event/paint barrier for the test. The same spec already has notifyTranscriptScrolled and waitForPaintedFrames helpers later in the eviction journey.

Expected closure

• Make scrollTranscriptTo notify the virtualizer explicitly and wait for the resulting painted frames.
• Preserve the test's exact tail mount, later eviction, focus-return, and selection-collapse assertions.
• Run the focused prompt-rail spec and full hosted Desktop e2e suite.

Found while validating #3789; the fix must remain in a separate PR.

[liugddx]

Fix proposed in #3863. The shared prompt-rail scroll helper now explicitly notifies the virtualizer and waits two painted frames before callers assert the mounted window; all exact eviction/focus assertions remain unchanged.

[liugddx]

Fix #3863 is exact-head green in hosted CI, including the full Desktop e2e suite. Keeping this issue open until the PR receives maintainer review and merges.