wcc: converge let-IDENT memcpy, IDENT-assign recv, nested struct call-recv onto structabisize

Three more wwstage cgen sites still used unrounded structnaturalsize where
cstage rounds via lu->size — pre-existing gate-blind cs!=ww latents the #169b
reviewer surfaced: cgenstmt N_LET struct-IDENT memcpy (let p2: T = p1; twin
cgen.c:7869), cgenexpr N_ASSIGN N_IDENT-lhs register RECV (s = mk(); twin
cgen.c:4700-4737), and cgenutil's nested struct N_CALL recv inside
cgstructlitfill (twin cgen.c:2121).

Converge all three onto structabisize, completing the same-class closure
started by #169 and continued by #169b. Also corrected the inline comment at
cgenutil.ww:3273-3286 that wrongly claimed fl->type->size was natural
(check.c:760 sets ABI). sretretsize at cgenutil.ww:1301 is gate-equivalent
natural and is left alone.

Probe 698 +3 rows (one per converged site) with cs==ww .s byte-cmp and a
pre-fix-rebuild discriminator. 990-997 byte-id hold.

selfhost/cmd/wcc/cgenexpr.ww

@@ -6739,9 +6739,14 @@ fn cgassign(c: *cgen, n: *node) void = {
 				if (lcsname.len > 0) {
 					let lcsi: *structinfo = structlookup(c, lcsname);
 					if (lcsi != nil) {
-						// si.totsize is slot-padded (rounded to 8);
-						// receive ABI needs the TYPE's natural size.
-						let lcnsz: i32 = structnaturalsize(lcsi);
+						// register RECV reads AX/DX/CX at 8-byte
+						// granularity — size via structabisize (cstage
+						// N_ASSIGN-IDENT branch sets sz = lu->size,
+						// cgen.c:4704; check.c:760 SSoT). The pre-
+						// #169 structnaturalsize shorts struct{i64,i32}
+						// (natural 12, ABI 16) to MOVQ+MOVL where
+						// cstage writes MOVQ+MOVQ.
+						let lcnsz: i32 = structabisize(lcsi);
 						if (n.op == tkind.TK_ASSIGN) {
 							if (n.rhs != nil
 							    && n.rhs.kind == nkind.N_STRUCTLIT) {