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/io/io.ww

@@ -5,14 +5,23 @@
 // value of read/write/close — Hare-shaped tagged unions instead of
 // errno-style integer sentinels.
 
-// eof — read past the end of the stream. NAMED-void so it's a
-// distinct variant tag from `void` (which would be "no result yet").
+// eof — read past the end of the stream. Hare uses the `done`
+// singleton for EOF; ww doesn't have `done` yet so we ship a
+// named-void variant tag.
 export type eof = void;
 
 // closed — operation attempted on a stream that has already been
-// closed. NAMED-void; same shape, different tag.
+// closed. ww-specific: Hare's io collapses this into the wider
+// errors union, but our stream vtable has no handle-ownership
+// semantics, so a distinct tag is honest. Named void.
 export type closed = void;
 
+// underread — an I/O handle hit eof partway through a fixed-size
+// read. Payload is the byte count actually delivered. Mirrors
+// Hare's `io::underread = !size`; ww uses i32 because the
+// underlying buffer-length type is i32 today.
+export type underread = !i32;
+
 export type stream = struct {
 	ctx:    *void,
 	read:   fn(s: *stream, buf: []u8) (i32 | eof | closed),