RUST.md, IRIS.md: record I3 -- List built and benchmarked, emulator step named

Ticks I3's box with the numbers (all flat across N as required),
updates "Where things stand", and adds IRIS.md's public-API entry for
List plus the fill-shaped-background lesson. The remaining emulator
comparison against transcript-bench.sh needs List wired into an actual
transcript/session screen (closer to I5's scope than I3's), so it's
recorded as the next step with the exact command rather than left
silently undone.

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

IRIS.md

@@ -8,6 +8,36 @@ capability that moved. Small and trivial changes do not go here.
 An entry gives the date, what changed, why, and a short before/after where
 it helps judge the change without the session that made it. Newest first.
 
+## 2026-09-05: `List`, a virtualised bottom-anchored list (RUST.md's I3)
+
+A new widget, `iris::widget::List` (`iris/src/widget/list.rs` -- read its
+module doc first), for the transcript's kind of screen: variable-height
+rows, keyed by a `u64`, composed only while visible, moved rather than
+re-laid-out on scroll, a scroll anchor that survives a row inserted above
+it, "more" sentinels at each end, and "hold the edge nearest the tap" when
+a row's height changes (`note_tap`, resolved in the layout pass).
+
+```rust
+let mut list = List::new(Axis::Y);
+list.push_back(ListRow::new(key, row_widget));   // O(1)
+list.push_front(ListRow::new(older_key, row));   // O(1), anchor unaffected
+list.set_more_before(Some(spinner_widget));      // sentinel, drawn at the edge
+list.note_tap(viewport_y);                       // before mutating a row's height
+let (top, bottom) = list.extent(key).unwrap();   // last frame's on-screen box, if visible
+```
+
+Built entirely out of existing primitives (`Painter::widget`/`widget_within`/
+`reposition`/`draw_twice`, and `draw_inner`'s own old-children diffing) --
+no new mechanism was added to the render core for it. One correctness
+lesson worth reading even for other widgets: a row that fills whatever
+region it is offered (`Rect`, `is_size_independent`) cannot be measured at
+a throwaway oversized region and then merely `reposition`ed into place --
+`reposition` only ever writes an offset, never a size, so the oversized
+primitive stays oversized. `List` fixes this by caching each row's real
+height once measured and placing an already-known row directly at its
+exact box; see `list.rs`'s `place` for the full reasoning and
+`a_fill_shaped_background_is_not_left_oversized` for the regression test.
+
 ## 2026-09-05: a second backend (android-view), and what moved to make room for it
 
 RUST.md's I2. Three changes a widget or app author would notice, all in