Keep still the end of a row nearest the tap, not the control pressed

Everything that opens now behaves alike. Touch a row's upper half and its
top edge holds, so it opens and closes downwards; touch the lower half and
the bottom edge holds, which is what the list does on its own. A group's
heading and the bar at its foot fall in the halves they already occupy, so
they keep the behaviour they had, and a single tool call -- one card, with
no bar -- gets the same choice for the first time: tapping low on an open
Bash card now shuts it downwards exactly as a group's bar does.

That makes the position of the tap the one mechanism, and RowEdge goes
away with the pair of hardcoded ends it existed to name. Controls report
where they were touched in root coordinates, which is all a control can
know -- a group is one row with a control at each end and calls in the
middle, and only the row knows where its own ends are -- and the row turns
that into an edge.

`clickableAt` is built on `clickable` rather than replacing it, so the
ripple and the click action assistive technology reads are unchanged; the
down position is observed on the initial pointer pass and nothing is
consumed.

Verified with ui-trace: on a collapsed group, a tap at y=1370 holds the
heading and one at y=1450 lets the row grow upward instead. On the same
nested call inside an open group, opening it from the group's upper half
holds the heading at 565 and from the lower half moves it to 296. ktfmt,
lint and 85 tests clean.

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

PLAN.md

@@ -624,9 +624,12 @@ 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
+   - Expanding a row keeps still **the end nearest the tap**: touch a row's
+     upper half and its top edge holds, so it opens downwards; touch its
+     lower half and the bottom edge holds, as the list does by default.
+     Which half, rather than which control, so that everything that opens
+     behaves alike whether or not it has a control at each end — a group's
+     heading and foot bar simply fall in the halves they already occupy. 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. The correction lives in the *layout* phase
      (`Modifier.holdTopEdge`): the measurement that discovers the row's new