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.
read → (i32 | eof | closed), write → (i32 | closed),
close → (void | closed). `type eof = void` and `type closed = void`
are exported as named-void variants — distinct nominal tags despite
identical (zero-byte) payloads.
No existing callers exercised the eof/closed sentinels (the smoke
test mimics the stream pattern with its own local types), so the
graduation is purely API shape — no callsite churn.
