Move a whole box at once, since that is what a move does

Profiling a move by cycles rather than by instructions says the cost is not
where the last session recorded it. In `apply_scalar` the `i64` division is
**0.00%** of cycles and the multiply 1.5%: the time is in `saturating_add`,
which is five instructions and no vector form for an `i32`, and a box that
only moved does eight of them. Asking for them one scalar at a time, each
behind a match on which kind of move this is, gives the compiler four short
sequences where it had four adds in a row to pair up.

So a translation is now asked for once for the whole region -- which is what
a translation is -- and the match happens once above it rather than per
scalar. `many` over 500 frames: 684M cycles to 660M, and 1,815,666,327
instructions to 1,742,553,104.

Cycle counts are worth trusting here, which is the other thing to keep: three
runs of one binary varied 0.23%. It is wall time that varies 2x on this
machine, not the counters, and instructions alone cannot see a stall.

Checked: fmt, clippy, 105 tests, all five shrinker cases at 300 seeds, and
`tabs`, `text` and `random` byte-identical at 1920x1200.

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

core/src/orientation/pos.rs

@@ -292,6 +292,15 @@ impl UiSpan {
     pub const fn len(&self) -> Len {
         self.end - self.start
     }
+
+    /// Both ends by the same amount, which is what moving a box without
+    /// changing its length does to every part of it.
+    pub const fn translated(self, by: Len) -> Self {
+        Self {
+            start: self.start + by,
+            end: self.end + by,
+        }
+    }
 }
 
 #[repr(C)]
@@ -302,6 +311,17 @@ pub struct UiRegion {
 }
 
 impl UiRegion {
+    /// Every part of the box by the same amount on each axis. Done to the
+    /// whole region rather than an end at a time, because that is what it is
+    /// -- and because four adds in a row are four adds, where four asked for
+    /// separately are four sequences.
+    pub const fn translated(self, x: Len, y: Len) -> Self {
+        Self {
+            x: self.x.translated(x),
+            y: self.y.translated(y),
+        }
+    }
+
     pub const FULL: Self = Self {
         x: UiSpan::FULL,
         y: UiSpan::FULL,

core/src/ui/render_state.rs

@@ -1090,6 +1090,14 @@ impl RegionRemap {
     }
 
     fn apply(&self, region: UiRegion) -> UiRegion {
+        // A box that only moved carries every part of itself by the same two
+        // amounts, and that is the common move. Asking it once for the whole
+        // region is what lets it be eight adds in a row rather than four
+        // sequences with a branch each -- measured, it is where the time in a
+        // move goes.
+        if let [AxisRemap::Translate(x), AxisRemap::Translate(y)] = self.axes {
+            return region.translated(x, y);
+        }
         UiRegion {
             x: self.axes[0].apply_span(region.x),
             y: self.axes[1].apply_span(region.y),