Stop a scroll asking a question it has already answered

A seventh sweep, over the parts no earlier round named: the tree generator
and the scenario harness, `Fixed`, the headless rig, and once more over the
commit the sixth sweep left, which was itself unreviewed.

`Scroll`'s content box is `answer_px.max(container_len)`, so a scroll whose
content fits has nothing to scroll through and `update_amt` has already put
`amt` at zero. The test choosing between the viewport and a scrolled span
asked `amt != ZERO || content_len != container_len`, where the first
disjunct can never decide it -- the same defect `b7b8d09` removed from the
line above, one operand over. A `debug_assert` of the implication held
across the whole suite, including every scrolling test.

`Fixed::to_scale` and its private `shift_round` arrived on this branch with
no caller and never got one; the only thing that called either was the test
written for them.

`Len::align` wrote `Len` arithmetic out a component at a time, around an
`at.px` that is always zero, where `Len::scale` and the `Add`/`Sub` beside
it say the whole rule in two lines. `LayoutLen::without_leftover` took
`self` where the `apply_leftover` its own doc calls the opposite reading of
the same value takes `&self`.

`run-headless.sh --resize` changed the output's mode but not `out_w`/`out_h`,
which is the extent `replay-touch` scales a recording against -- so
`--resize` with `--replay` put every sample of the gesture somewhere else
and still finished like a run that worked. Both come from one function now.

The generator's plan/build split stranded a comment: "a row takes the height
it is given" describes the size rule `build` derives from `dir`, and it was
left above the `gap` draw, which is the one line it is not about and which
does consume randomness.

Format, clippy with and without layout-diagnostics, and the suite (131 + 19
+ 13 + 4) are clean. The cold dump over 400 depth-5 trees is byte-identical
to b7b8d09 across all 34,492 boxes, and all three seed scans pass: 400 at
depth 5 in 62.75s, 1,000 at depth 6 in 162.37s, 2,000 at depth 4 in 305.25s.

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

core/src/orientation/align.rs

@@ -151,13 +151,15 @@ impl Vec2 {
 }
 
 impl Len {
+    /// This length placed in the box it is measured in: the alignment names a
+    /// point along that box, and the two ends are that point less the part of
+    /// the length falling before it and plus the part falling after.
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
-        let rest = Rel::ONE.sub(rel);
         let at = Len::from_parts(rel, Px::ZERO);
         UiSpan {
-            start: Len::from_parts(at.rel.sub(self.rel.mul(rel)), at.px.sub(self.px.mul(rel))),
-            end: Len::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
+            start: at - self.scale(rel),
+            end: at + self.scale(Rel::ONE.sub(rel)),
         }
     }
 }