Float array-element stores (array-literal init, [v...] repeat-fill, and
arr[i]=v) now route from X0 via MOVSS/MOVSD in both stages; the AX path
stored the raw double low-bits, garbage for f32 (f64 worked by accident).
A clobbering call-index (a[geti()]=v) loses the X0 value — deferred to #125.