Prompt rail loses the active Turn highlight while scrolling the transcript #4415

What happened

The prompt anchor rail on the right side of the Desktop transcript does not reliably follow the Turn currently being read.

After scrolling from the final Turn back to the first Turn, the rail can show no active tick at all. The expected behavior is that exactly one rail tick remains highlighted and tracks the visible Turn throughout manual transcript scrolling.

The failure has two contributing conditions:

• The rail writes data-active and aria-current imperatively while React also renders those attributes. A later transcript or virtual-window render can remove the imperative attributes while the active-Turn ref still suppresses a repair.
• The intersection observer only watches the top reading band. During virtual-window transitions or at a Turn boundary, that band can contain only a spacer or gap even though a Turn is visible elsewhere in the scrollport.

How to reproduce

1. Open Maka Desktop and select a session with at least three Turns and enough content to overflow the transcript.
2. Scroll the transcript to the final Turn.
3. Scroll manually back to the first Turn.
4. Inspect the prompt anchor rail on the right.

Actual: no rail tick may have aria-current="true" / the active visual treatment.

Expected: the first Turn's tick is highlighted after reaching the top, and the active tick changes as the visible Turn changes.

Environment

• Maka commit: 28188e651
• OS: macOS 15.6 (24G84)
• Surface: Desktop
• Node.js: v24.19.0

Logs, screenshots, or additional context

The transcript is virtualized, so the regression should be covered in the real-window prompt-rail E2E fixture by scrolling from the last prompt to the first and asserting that exactly the corresponding tick is current at each extreme.