w6c_ww: emit a bare str/slice global zero-header once, keyed on type kind (fix #10 part b)

Post-#1, size(str) == size(slice) == 24. emitletdataw's str arm
(~cgen.ww:2074) and slice arm (~cgen.ww:2139) were sequential `if`s
gated on SIZE alone, so a bare 24-byte global matched BOTH and BOTH
fired the no-rhs zero fallback — two `DATAW main.g` rows. cstage
discriminates on type kind (let_isstr/let_isslice, cgen.c:1026/1036)
and emits one; the link+run is correct either way, so the divergence
was byte-id-visible only.

Gate the two arms on the declared type kind via the new letdeclkind
helper (d.lhs.type_, TY_NAMED-peeled — the resolvewalk-stamped
type-expression node), mutually exclusive: a 24B global now hits one
arm. Falls to the str arm when unstamped, where the zero-init bytes are
identical, so byte-id holds for that case too.

cstage already correct — no change. New test 686 (5 runtime rows + 5
byte-id rows) pins single-emit + cs==ww.

Makefile

@@ -243,6 +243,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_arr_strslice_elem \
 	  $(BIN)/test_arr_tagged_elem \
 	  $(BIN)/test_arr_infer_len \
+	  $(BIN)/test_slice_str_global_zero \
 	  $(BIN)/test_dot_str_chained_arg \
 	  $(BIN)/test_dot_slice_arg \
 	  $(BIN)/test_dot_tagged_source \
@@ -575,6 +576,12 @@ $(BIN)/test_arr_tagged_elem: test/wcc/685_arr_tagged_elem.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_slice_str_global_zero: test/wcc/686_slice_str_global_zero.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_dot_str_chained_arg: test/wcc/692_dot_str_chained_arg.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \