wcc: struct-return float fields via SSE return regs (#171a)

The RETURN twin of #165: a qualifying float-struct was returned GP-only
(struct{f64,f64} in AX/DX instead of X0/X1) — value-correct via GP transport
but not SysV register-class conformant. Route each float eightbyte through the
SSE return cursor (X0/X1) and each integer eightbyte through GP (AX/DX) via
independent cursors, at the struct-return SEND and RECV, both stages, reusing
struct_float_class verbatim. Closes the temporary tuple-SSE/struct-GP
divergence opened across #164/#165.

A qualifying struct has >=1 lone f64 so maxalign is 8 and the ABI slot is an
8-multiple — no sub-8 tail — so #169's sized tail is unreachable here and the
integer eightbyte uses a full MOVQ (cstage agrees, proven by the f64i32
cs==ww byte-id). f32 / multi-float-per-eightbyte stays GP (deferred #171b);
>16B stays sret.

Gate-blind and value-correct, so the discriminator is the SEND/RECV register
class (MOVSD X0/X1 vs MOVQ AX/DX) — covered by probe 946_structret_run.

Makefile

@@ -355,6 +355,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_tuprecv_f64_run \
 	  $(BIN)/test_tupparam_run \
 	  $(BIN)/test_structparam_run \
+	  $(BIN)/test_structret_run \
 	  $(BIN)/test_floats_run \
 	  $(BIN)/test_size_type_run \
 	  $(BIN)/test_types_sizelim_run \
@@ -1278,6 +1279,11 @@ $(BIN)/test_structparam_run: test/wcc/946_structparam_run.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_structret_run: test/wcc/946_structret_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 sizelint:
 	@sh tools/sizelint