Name the values layout carries, and say what a span's slot is

`along` said nothing about what it did. It is `Span::slot` now: the
stretch of the row between two distances from where the span starts
laying out, as a span of its own box, with the mirror for a negative
direction in one place. `far` is `row`, which is what the comment above
it already called it, and `shares` is `has_room` beside the
`any_leftover` it was folded into. `reached` now guards on the leftover
weight it divides by rather than on the numerator that happened to be
zero with it.

The pairs layout returns are named rather than positional: `Answer`
{size, holds} and `Drawn` {answer, drawing_holds} replace
`(Size, LayoutHolds)` and a three-tuple with two `LayoutHolds` in it,
which was the one shape the cold dump exists to catch. `try_reuse`
answers `bool` rather than `Option<()>`, and the four hand-written
copies of `move_idx != parent_move` are `ActiveData::is_region_node`.

`AXES` was declared in three modules; it is `Axis::BOTH`. `rel_min`,
`rel_max` and the unused `select_len` are gone -- `ZERO` and `FULL`
already said those. Three doc comments sat on `impl` blocks instead of
the single method inside them. `reposition` and `redepth` walked their
children by index, looking the parent up again per child; both take the
list and put it back. `Scroll`'s `fixed` and `fixed_len` are
`answer_px` and `answer_is_px`, which says which one is the length.

fmt, workspace clippy under `-D warnings` with and without
`layout-diagnostics`, and the workspace tests are clean. The cold dump
over 400 depth-5 trees is byte-identical to `6c84b6f`: 34,492 boxes,
no seed moved.

core/src/orientation/pos.rs

@@ -163,14 +163,6 @@ impl Len {
         Self::from_parts(Rel::ZERO, Px::from_f32(px))
     }
 
-    pub const fn rel_min() -> Self {
-        Self::ZERO
-    }
-
-    pub const fn rel_max() -> Self {
-        Self::FULL
-    }
-
     pub const fn max(&self, other: Self) -> Self {
         Self {
             rel: self.rel.max(other.rel),
@@ -213,10 +205,6 @@ impl Len {
         })
     }
 
-    pub fn select_len(&self, len: Len) -> Self {
-        len.within_len(*self)
-    }
-
     pub const fn flip(&mut self) {
         self.rel = Rel::ONE.sub(self.rel);
         self.px = self.px.neg();