wcc: f32 array-element store from X0 (#122)

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.

selfhost/cmd/wcc/cgenexpr.ww

@@ -4206,6 +4206,24 @@ fn cgassign(c: *cgen, n: *node) void = {
 						emitline("\tMOVQ\tCX, 16(BX)\n");
 						return;
 					};
+				// float element → store FROM X0 (MOVSS/MOVSD): cgexpr
+				// left the value in X0. X0 survives the index/base
+				// eval only for literal/local-var indices; a call-
+				// index (a[geti()]=v) clobbers X0 and loses the value
+				// — this path does not spill X0 across the eval,
+				// unlike the *p=v float deref store. Deferred to #125.
+				// For f32 the #104 CVTSD2SS narrowing only touches X0,
+				// so the AX store below would write raw double low-
+				// bits, garbage for f32 (#122, mirrors cstage cgen.c
+				// arr[i]= float store).
+				if (isfloattype(c, elemtn)) {
+					let fmov: str = "MOVSD";
+					if (isf32type(c, elemtn)) { fmov = "MOVSS"; };
+					emitline("\t");
+					emitline(fmov);
+					emitline("\tX0, (BX)\n");
+					return;
+				};
 				let isop: str = tnodestoreop(c, elemtn, esz);
 				emitline("\t");
 				emitline(isop);