Open a row downwards, from whichever end was pressed

Tapping a group's heading used to send that heading up off the top of the
screen and fill the space above it, so the calls appeared on the far side
of the control that produced them. The transcript is laid out from the
bottom, so every row's bottom edge is what the list holds still and all
growth goes upward.

The rule now is that the end the reader pressed is the end that must not
move. A heading anchors the top, so the row opens downwards under it; the
bar at the foot of an open group anchors the bottom, so shutting it from
there leaves what follows the group where it is -- which is what already
happened, but by accident of the layout rather than on purpose, and would
have been lost the moment anything else changed.

Bottom is the list's own behaviour and costs nothing. Top is measured
rather than calculated: only the layout knows how tall an open group is,
so `toggleAnchored` reads where the top edge was, lets the change land,
and scrolls by however far it moved.

Applied to every row that opens, not just groups -- a lone tool call and a
peer message are the same gesture, and one of them opening the other way
would be the odder for it.

PLAN.md

@@ -624,6 +624,13 @@ 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