Must not wrap io.EOF

From the Go docs:

```go
// EOF is the error returned by Read when no more input is available.
// (Read must return EOF itself, not an error wrapping EOF,
// because callers will test for EOF using ==.)
// Functions should return EOF only to signal a graceful end of input.
// If the EOF occurs unexpectedly in a structured data stream,
// the appropriate error is either [ErrUnexpectedEOF] or some other error
// giving more detail.
var EOF = errors.New("EOF")
```

however, `Conn.reader` (and probably others) wrap io.EOF:

```go
func (c *Conn) reader(ctx context.Context) (_ MessageType, _ io.Reader, err error) {
defer errd.Wrap(&err, "failed to get reader")

// Wrap wraps err with fmt.Errorf if err is non nil.
// Intended for use with defer and a named error return.
// Inspired by https://github.com/golang/go/issues/32676.
func Wrap(err *error, f string, v ...any) {
if *err != nil {
*err = fmt.Errorf(f+": %w", append(v, *err)...)
}
}
```

This leads to errors such as https://github.com/hashicorp/yamux/pull/167.

I'd propose special-casing `Wrap` to check for `io.EOF` and would be happy to open a PR.

Start at errd.Wrap and its use in Conn.reader, then review the Go io.EOF documentation cited in the issue. Reproduce the wrapped-EOF case and check the project's existing tests; done means EOF remains directly comparable while other errors retain context.

Written by the indexing model from the issue text.