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>

src/widget/position/scroll.rs

@@ -45,13 +45,14 @@ impl Widget for Scroll {
             painter.holds(self.axis, Px::MIN..=left);
         }
 
-        // Content that fills the viewport and has not been scrolled is the
-        // viewport, and is handed back as it came. Writing the same box as
-        // its own length in pixels is the same box in another form, and the
-        // two do not round alike: a part centred in `rel 1` lands a step from
-        // one centred in `px 900`, since halving a difference is not halving
-        // each part of it.
-        let content = match self.amt != Px::ZERO || self.content_len != self.container_len {
+        // Content that fills the viewport is the viewport, and is handed back
+        // as it came -- it has nothing to scroll through, so the clamp above
+        // has already put `amt` at zero. Writing the same box as its own
+        // length in pixels is the same box in another form, and the two do
+        // not round alike: a part centred in `rel 1` lands a step from one
+        // centred in `px 900`, since halving a difference is not halving each
+        // part of it.
+        let content = match self.content_len > self.container_len {
             true => {
                 let start = Len::from_parts(Rel::ZERO, -self.amt);
                 UiSpan::new(start, start.offset(self.content_len)).shifted_desc()