lib/fnmatch: explicit '\\': u8 cast; fnmatch graduates to the byte-id gate

wwstage over-rejects harec's flexible-rune-const narrow (rune lit
promotes into any int whose range fits — harec check.c:1910,
types.c promote_flexible); explicit cast until that promotion lands
in ww (#29, revert to bare '\\' then). With the enum-as fix fnmatch
is byte-identical across stages: 989 #59.14 graduates
M_WWREJECT -> M_ID, transferring 972's cstage 8/8 behavior to the
provably-identical wwstage binary — the dark-regression gate. (#29)

lib/fnmatch/fnmatch.ww

@@ -123,7 +123,8 @@ fn pat_next(pat: str, pos: *i32, fl: flag) (u8 | star | question | bracket | end
 			*pos += 1;
 			return c2;
 		};
-		return '\\';
+		// #29: wwstage over-rejects the implicit flexible-rune-const narrow (harec promote_flexible accepts); explicit cast until flexible-const promotion lands — revert to bare '\\' then.
+		return '\\': u8;
 	};
 	return c;
 };