lib+test: unify errors to Hare named-void tagged-union

Five tags from ref/hare/errors/common.ha — invalid, noaccess, noentry,
exists, unsupported — all !void. lib/io keeps eof/closed as plain void
(no Hare analogue for ww's singleton-style done) and adds underread.
Drops errors.equal/isnil and the old str-sentinel surface.

lib/bufio/bufio.ww

@@ -40,8 +40,9 @@ export fn readbyte(b: *buf) (i32 | void) = {
 
 // Distinct alias so `(str | linerr)` has two variant types the
 // tagged-union machinery can keep apart at the tag level. The error
-// variant carries a short description; callers compare with errors.equal
-// or just inspect by length.
+// variant carries a short description; callers inspect by length.
+// Placeholder until bufio graduates to io.stream + named-void tags
+// (eof / closed / underread).
 type linerr = str;
 
 // readline — Hare-style fallible line read (name transliterated from