Revert "Open a row downwards, from whichever end was pressed"

This reverts commit f4d4c82. The anchoring it added made the transcript
jump on every expand and collapse, and left the list snapping back to the
bottom when somebody scrolled up, which is worse than the upward-opening
it was meant to fix.

Two things to look at when this is retried. `LazyListItemInfo.offset` in a
`reverseLayout` list is not obviously the coordinate space this assumed,
so `offset + size` may have been measuring the bottom edge -- the one the
list already holds -- rather than the top. And the anchoring scroll ran in
a coroutine that could still be pending when the reader started dragging;
`scrollBy` takes the default mutation priority, so it cancels that drag.

Verify the next attempt with `uiautomator dump` -- node bounds in device
pixels, before and after a toggle -- rather than by eye from screenshots.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

PLAN.md

@@ -624,13 +624,6 @@ dev-updater (Kotlin 2.4.x, CMP 1.11.x, JDK 21). Screens:
      output; a spinner while `ToolStart` has no matching `ToolEnd`).
    - Question cards inline: option buttons for AskUserQuestion, allow/deny for
      permissions, free-text where allowed.
-   - Expanding a row **opens downwards**: whichever end the reader pressed —
-     a group's heading or the bar at its foot — is the end that stays put,
-     and the row grows away from it. The transcript is laid out from the
-     bottom, so a row's bottom edge is anchored for free and the top one
-     has to be arranged; `toggleAnchored` measures the move and scrolls it
-     back (2026-08-30, asked for after groups opened upwards and sent their
-     own heading off the top of the screen).
    - Input bar: text, attach (camera/gallery/file), send — **always enabled**;
      mid-run sends become steering messages.
    - Top bar: model chip (tap to change), stop button while running, token