wcc: struct-param float fields via SSE arg regs (#165)

struct params were passed GP-only, so a struct{f64,f64} argument landed in
DI/SI instead of X0/X1 — value-correct for internal ww calls (the bits
round-trip) but not SysV register-class conformant. Add a per-eightbyte
classifier (struct_float_class) routing a qualifying struct's float eightbytes
through the SSE arg cursor, reusing #163's dual-cursor plumbing and #164's
field classification. A struct qualifies only when every eightbyte is
pure-integer or a lone f64 exactly filling it (and >=1 f64); anything else —
any f32, multiple floats per eightbyte, a straddling or aggregate field —
falls back to the unchanged GP path (f32 sub-eightbyte packing deferred #165b).
Both stages' predicates are alias-aware and identical in coverage.

Gate-blind and value-correct either way, so the discriminator is the callee's
receive instruction (MOVSD vs MOVQ), scoped per-function — covered by probe
946.

Makefile

@@ -354,6 +354,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_f32arg_run \
 	  $(BIN)/test_tuprecv_f64_run \
 	  $(BIN)/test_tupparam_run \
+	  $(BIN)/test_structparam_run \
 	  $(BIN)/test_floats_run \
 	  $(BIN)/test_size_type_run \
 	  $(BIN)/test_types_sizelim_run \
@@ -1272,6 +1273,11 @@ $(BIN)/test_tupparam_run: test/wcc/905_tupparam_run.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_structparam_run: test/wcc/946_structparam_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 sizelint:
 	@sh tools/sizelint