test: 953 f64 cross-module return cast/arg cs==ww byte-id + cstage run (#98/#101)

Regression net with two dimensions per row because cstage is correct
before and after the fix — a cstage-only probe is gate-blind to a
wwstage-only divergence:
  (a) cstage `ww build` + run, asserting the truncated exit code;
  (b) w6c vs w6c_ww `.s` cmp, FAILING on any rule-10 divergence.

Rows cover #101 (`mod.g(): i32`, neg + both truncation directions) and
#98 (`dbl(mod.g())` forwarding an imported f64 call-result as an f64
arg). Verified: dimension (b) FAILS on master 6f8b658 (pre-fix wwstage
emits MOVSXD/integer-PUSHQ) and PASSES post-fix (byte-identical);
dimension (a) passes on both, confirming cstage was always correct.

Makefile

@@ -328,6 +328,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_siphash_run \
 	  $(BIN)/test_checked_run \
 	  $(BIN)/test_f64cgen_run \
+	  $(BIN)/test_f64crossmod_run \
 	  $(BIN)/test_floats_run \
 	  $(BIN)/test_bufio_run $(BIN)/test_random_run
 
@@ -1070,6 +1071,11 @@ $(BIN)/test_floats_run: test/wcc/952_floats_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_f64crossmod_run: test/wcc/953_f64crossmod_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 sizelint:
 	@sh tools/sizelint