fix(rust): a field that names its own struct needs a Box (+3) - #3378

Closes #3377

Chosen with tri one-away, not guessed. It reported recursive type X has infinite size as the sole error of 3 specs whose count is exact; the repair moved exactly those 3. First fix in five passes to move the column.

pub const KDNode = struct { left : "?KDNode", ... }
  ->  pub left: Option<KDNode>          inside  pub struct KDNode
rustc: recursive type `KDNode` has infinite size ... insert some indirection

There is no version of this that compiles without indirection, so nothing is being chosen about representation — it is the difference between output and no output. Box is the ownership-preserving one of the three names rustc offers.

Two shapes, and the second corrects a claim of mine. Option<Name> is kd_tree. [Option<Name>; N] is octree and quadtree — and an earlier pass of mine wrote octree.t27 off as "genuinely infinitely sized". It is not: an array is inline storage, only the element needs the box, and [Option<Box<OctNode>>; 8] is finite.

Measured, two pinned binaries built from the same commit in one pass, 650 specs: 357 → 360, +3, zero regressions.

Nine structs in the corpus name themselves; three were one-away and are now green. Narrow on purpose: Vec<Name> already carries its indirection and is untouched, and a shape this does not recognise keeps its current output rather than getting a guess.