Carry a box in a move slot, not a translation

A slot now holds the box its contents are placed within, in the coordinates
of the slot it names, and `prelude.wgsl` composes the chain with `within`
instead of adding a delta. A translation is the special case where the box
has its parent's relative extent, so every caller passes
`UiRegion::FULL.offset(delta)` and nothing changes on screen yet: 42 tests
pass and `tabs` at 1920x1200 is byte-identical.

`Moves::resolve` takes the region to compose rather than returning a sum, so
the CPU walk is the same operation the shader performs.

Measured against the translate slot on the same binary with
`tests/chain_cost.rs`, 200k instances: +0.6% at depth 1, +0.5% at 2, +0.8% at
4, then +9.6% at 8 and +32.2% at 64. Free at the depth opt-in slots produce,
which is the next commit; the per-level cost was always the dependent load
rather than the arithmetic.

The identity is `UiRegion::FULL` rather than zero, which `MoveOffset`'s
comment says beside the `Zeroable` that `Pod` requires: a zeroed entry is a
box of no extent and collapses its subtree to a point.

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

core/src/ui/mod.rs

@@ -1,7 +1,7 @@
 use crate::{
-    Mask, MoveIdx, MoveOffset, PrimitiveRegistry, TextData, Textures, WeakWidget, WidgetId,
-    Widgets,
-    util::{Arena, Id, TrackedArena, Vec2},
+    Mask, MoveIdx, MoveOffset, PrimitiveRegistry, TextData, Textures, UiRegion, WeakWidget,
+    WidgetId, Widgets,
+    util::{Arena, Id, TrackedArena},
 };
 
 /// How far the shader will walk a move chain. It bounds a malformed cycle
@@ -46,26 +46,27 @@ impl Moves {
         self.arena.remove(Id::preset(idx.idx() as u32));
     }
 
-    /// Sets a slot's translation, in physical pixels, relative to its parent.
-    pub fn set(&mut self, idx: MoveIdx, delta: Vec2) {
+    /// Sets the box a slot's contents are placed within, itself given in the
+    /// coordinates of its parent slot.
+    pub fn set(&mut self, idx: MoveIdx, region: UiRegion) {
         let entry = self.arena.get_mut(Id::preset(idx.idx() as u32));
-        if entry.delta != delta {
-            entry.delta = delta;
+        if entry.region != region {
+            entry.region = region;
             self.changed = true;
         }
     }
 
-    /// The translation a primitive in `idx` has accumulated, which is the
-    /// same walk the vertex shader does.
-    pub fn resolve(&self, idx: MoveIdx) -> Vec2 {
-        let mut total = Vec2::ZERO;
+    /// Composes a region held in `idx`'s coordinates down the chain, which is
+    /// the same walk the vertex shader does.
+    pub fn resolve(&self, idx: MoveIdx, local: UiRegion) -> UiRegion {
+        let mut region = local;
         let mut at = idx;
         for _ in 0..CHAIN_LIMIT {
             if at == MoveIdx::NONE {
-                return total;
+                return region;
             }
             let entry = self.arena[at.idx()];
-            total += entry.delta;
+            region = region.within(&entry.region);
             at = entry.parent;
         }
         debug_assert!(
@@ -73,7 +74,7 @@ impl Moves {
             "a move chain longer than {CHAIN_LIMIT} resolves to the wrong place, \
              and the shader stops at the same depth"
         );
-        total
+        region
     }
 
     pub fn entries(&self) -> &[MoveOffset] {