w6c+wwstage: zero high pad words on scalar/float widen into a >16B tagged union (#227)

cg_widen_tagged_store (cmd/w6c/cgen.c) and the wwstage twin cgwidentaggedstorebp (selfhost/cmd/wcc/cgenutil.ww) wrote only the tag (slot+0) and value (slot+8) in their scalar and float arms, leaving the high pad words (slot+16..sz) as stack garbage on the BP/let/assign/return-scratch path, which never pre-zeroes. A passthrough return or u8-reinterpret of a narrow scalar/float widened into a >16B union (fmt's field = (...formattable | *mods) is 32B via the str variant) then read that garbage. Both stages were wrong identically, so the byte-id gates stayed green while the runtime truncated; fmt's spread-union scalar widen is the first real consumer. Both arms now tail-zero slot+16..sz (gated size>16), mirroring the tagged-subset/struct tail-zeros and keeping the stages byte-identical (rule 10). Adds runtime test 793; regenerates w6c/wwdump combined.ww. fmt byte-id graduation still awaits the other residual, #226 (io.read nominal-remap).

Makefile

@@ -340,6 +340,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_xmod_alias_struct_collide_run \
 	  $(BIN)/test_xmod_variant_match \
 	  $(BIN)/test_spread_variant_match \
+	  $(BIN)/test_widen_pad_zero_run \
 	  $(BIN)/test_named_ptr_alias_variant_widen \
 	  $(BIN)/test_single_field_struct_zeroinit \
 	  $(BIN)/test_structvariant_largeunion_return \
@@ -790,6 +791,12 @@ $(BIN)/test_spread_variant_match: test/wcc/792_spread_variant_match.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_widen_pad_zero_run: test/wcc/793_widen_pad_zero_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/w6c_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 # #15: widening a bare *vtable into a NAMED-alias variant (`stream` =
 # *vtable) of `(file | stream)` must compute the right tag, not default
 # to tag 0. Both-stage byte-id + runtime, plus a degenerate-ambiguity