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/render/data.rs

@@ -65,13 +65,16 @@ impl MoveIdx {
     }
 }
 
-/// One link of the chain a primitive's position is resolved through: a
-/// translation in physical pixels, and the slot it is relative to. Moving a
-/// subtree writes its own slot and nothing else.
+/// One link of the chain a primitive's position is resolved through: the box
+/// its contents are placed within, given in the coordinates of the slot it
+/// names. Moving or resizing a subtree writes its own slot and nothing else.
+///
+/// The identity is `UiRegion::FULL`, not zero: a zeroed entry is a box of no
+/// extent, which collapses everything under it to a point.
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct MoveOffset {
-    pub delta: Vec2,
+    pub region: UiRegion,
     pub parent: MoveIdx,
 }
 
@@ -81,7 +84,7 @@ unsafe impl bytemuck::Zeroable for MoveOffset {}
 impl MoveOffset {
     pub fn root(parent: MoveIdx) -> Self {
         Self {
-            delta: Vec2::ZERO,
+            region: UiRegion::FULL,
             parent,
         }
     }