iris: a List gives back its overscroll in the frame that found it

The last place in iris that corrected itself on a later frame, and the
item docs/IRIS_TODO.md carried from the Scroll change. Iris's rule:
"nothing in the framework should ever self heal because it should not be
drawn incorrectly in the first place. If you need 2 draws to get
something into the correct position then that should happen within the
same frame."

`clamp_to_content` measured the gap past the end of the content from the
edges the walk had just placed, wrote it to the anchor and asked for
another frame -- so one frame was drawn with the list past its own end,
and a fling that had already stopped was not going to ask for the frame
that fixed it. Now the walk outward from the anchor is `List::lay_out`,
`overscroll_gap` is a pure measurement of the same gap (no painter, no
redraw handle), and `draw` moves the anchor and walks a second time
inside the same frame.

One further pass always settles it: the gap comes from the edges the
first walk placed, so moving the anchor by it puts that edge exactly on
the viewport's, and the opposite end can only open a new gap when the
content is shorter than the viewport, which `overscroll_gap` declines to
touch. The second walk is paid only on an overscrolled frame and re-offers
every row the same cached-height box at a new offset, which `draw_inner`
dispatches as an O(1) move.

`Painter::draw_again` had no other caller and is removed with it, so the
framework no longer offers a way to ask for a corrective frame at all.

Simplification in the same change: a placement is one pinned edge plus a
height, so `Placement::edges(height)` gives the box and `place`'s
top-known and bottom-known cases stop being two copies of the same
arithmetic -- four match arms down to two.

Four tests draw no settling frame on purpose and fail without the change:
`fling_toward_the_start_stops_at_the_first_row` and the new
`scrolling_past_the_start_is_given_back_in_the_same_frame` (list.rs), and
`scrolling_past_the_first_row_settles_on_it` /
`scrolling_past_the_last_row_settles_on_it` (layer 1, top_edge.rs).

Verified: cargo fmt --check, clippy --workspace --all-targets clean,
cargo test --workspace and ./run-tests.sh green, the phone-shaped
headless window replaying flick-120hz.touch draws the transcript
correctly, and the arm64 release APK builds.

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

docs/DECISIONS.md

@@ -5,6 +5,21 @@ they can be judged and reversed later. Detail lives in RUST.md (and IRIS.md
 for iris API changes); this file is only the summary. Newest first. Items
 marked **DEFERRED** are ones the agent chose not to decide alone.
 
+## 2026-09-08 (later still: the list's overscroll clamp, in frame)
+
+Finishes the item the previous entry deferred. IRIS.md has the account.
+
+- **`List` lays out a second time within the frame** when its walk lands
+  off the end of the content, instead of writing the correction to the
+  anchor and asking for another frame. The extra walk is paid only on an
+  overscrolled frame, and it is mostly O(1) moves.
+- **`Painter::draw_again` is removed**, `List` having been its only
+  caller -- so the framework no longer offers a way to ask for a
+  corrective frame at all.
+- **`List::place`'s top-known and bottom-known cases are one path**
+  (`Placement::edges`), which is the "write the logic once" rule applied
+  to two symmetric directions rather than a behaviour change.
+
 ## 2026-09-08 (later: a scroll area measures and places in one frame)
 
 From Iris's phone report about the composer's padding while typing