Fixes during debugging:
- Variable scope: loop's closing parens closed the let* prematurely,
making children/is-row/pr/pb undefined in own-size calculation
- gap NIL bug: make-layout-node passed :gap nil (from &key default)
to make-instance, overriding :initform 0 → (* nil ...) crash
- Child order: push (LIFO) in add-child reversed children order;
changed to nconc (FIFO), removed the compensating reverse
- Fixed distribute-sizes to base all children from their fixed size
then apply grow/shrink on top, instead of treating fixed-size
children as non-participating
