iris: a widget's move slot has one owner -- move_applied + repositioned

`mov` accumulates a delta onto the slot and `reposition` overwrote it, and
both legitimately land on one widget in one frame: `List::place`'s
Bottom-known branch offers a row a same-size box that has moved (`mov`),
then corrects the placement inside it when the row's cached height no
longer matches what the row reports (`reposition`). That is what a wrapped
transcript row hit, and what the `move_applied == ZERO` debug assert was
standing in for -- an assert against a case that happens is not a
guarantee, it is a crash.

The slot means `move_applied + repositioned` now, both halves recorded on
`ActiveData`, so `reposition` adds the move rather than dropping it and
stays idempotent. The assert it replaces is a `debug_assert_eq!` that the
slot still holds that sum on entry -- i.e. that nothing but those two ever
wrote it.

Test: `a_widget_moved_by_its_parent_and_then_placed_inside_it_lands_at_the_placement`,
which draws the child at the offered position (-100px) rather than the
placement (100px) without the fix. Verified against the `.wrap(true)`
repro from docs/IRIS_TODO.md (draws correctly, no panic) and an emulator
bench run with assertions live.

iris/core/src/ui/active.rs

@@ -53,4 +53,17 @@ pub struct ActiveData {
     /// after a finger pan (2026-09-06). Reset to zero whenever the widget
     /// is really redrawn, since `draw_inner` zeroes the slot then too.
     pub move_applied: Vec2,
+    /// The offset the last `Painter::reposition` placed this widget's
+    /// content at *within* `region`, in window pixels. The move slot has
+    /// exactly one owner and one meaning:
+    /// `move_offsets[move_slot] == move_applied + repositioned`. `mov`
+    /// adds to the first, `reposition` overwrites the second (it
+    /// recomputes `from` afresh every call, so repeating it must land on
+    /// the same answer rather than drifting), and both then rewrite the
+    /// slot from the sum -- which is what lets a parent both move a child
+    /// with its own layout and place it inside that moved region in one
+    /// frame. `List::place`'s Bottom-known branch does exactly that once a
+    /// row's blocks wrap. Reset to zero on a real redraw, with
+    /// `move_applied` and the slot itself.
+    pub repositioned: Vec2,
 }

iris/core/src/ui/render_state.rs

@@ -380,6 +380,7 @@ impl UiRenderState {
             move_slot,
             own_mask,
             move_applied: Vec2::ZERO,
+            repositioned: Vec2::ZERO,
         };
 
         // remove old children that weren't kept
@@ -442,12 +443,8 @@ impl UiRenderState {
         let Some(active) = self.active.get(&id) else {
             return;
         };
-        debug_assert!(
-            active.move_applied == Vec2::ZERO,
-            "widget {id:?} is both moved by its parent's own layout (`mov`) and repositioned \
-             within it; the two write the same slot with different conventions -- see \
-             `ActiveData::move_applied`"
-        );
+        let move_applied = active.move_applied;
+        let repositioned = active.repositioned;
         let from = active
             .size
             .to_uivec2(self.density)
@@ -457,8 +454,27 @@ impl UiRenderState {
         let from_px = from.top_left().to_abs(self.output_size);
         let to_px = to.top_left().to_abs(self.output_size);
         let delta = to_px - from_px;
+        // Not `delta` alone: a parent may have `mov`ed this widget to a
+        // region that itself moved earlier in the same frame, and that
+        // part of the slot is `move_applied`'s, not this call's. Writing
+        // `delta` on its own dropped it and put the content back at the
+        // pre-move position. `from` is computed against `active.region`,
+        // which `mov` already updated, so `delta` is purely the placement
+        // inside the region and the two summands never overlap.
         let entry = rsc.ui_mut().move_offsets.get_mut(slot);
-        entry.delta = [delta.x, delta.y];
+        debug_assert_eq!(
+            entry.delta,
+            [
+                move_applied.x + repositioned.x,
+                move_applied.y + repositioned.y
+            ],
+            "widget {id:?}'s move slot was written by something other than `mov`/`reposition`; \
+             the slot is theirs and means `move_applied + repositioned` -- see `ActiveData`"
+        );
+        entry.delta = [move_applied.x + delta.x, move_applied.y + delta.y];
+        if let Some(active) = self.active.get_mut(&id) {
+            active.repositioned = delta;
+        }
         self.mov_count += 1;
     }