changesReader: `Only GET allowed`

Trying to convert nano.feedUpdated to nano.db.use().changesReader ...

```js
let feed = nano.db.use('_db_updates').changesReader.get({ since: lastStopped, include_docs: true });
// same with .start

feed.on('error', (err) => {
console.error('changes feed error:\n', err);
})
```

changes feed error:
```
Error: Only GET allowed
at responseHandler (.../node_modules/nano/lib/nano.js:203:20)
at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
{
scope: 'couch',
statusCode: 405,
request: {
url: 'http://localhost:5984/_db_updates/_changes?feed=longpoll&timeout=60000&since=....&limit=100&include_docs=false',
method: 'post',
headers: {
accept: 'application/json',
'user-agent': 'nano/11.0.4 (Node.js v25.8.2)',
'Accept-Encoding': 'deflate, gzip',
Authorization: 'XXXXXXX',
'content-type': 'application/json'
},
credentials: 'include',
body: '{}',
redirect: 'error',
signal: AbortSignal { aborted: false },
bodyTimeout: 0
},
headers: {
uri: 'http://localhost:5984/_db_updates/_changes?feed=longpoll&timeout=60000&since=....&limit=100&include_docs=false',
statusCode: 405
},
errid: 'non_200',
description: 'Only GET allowed',
error: 'method_not_allowed',
reason: 'Only GET allowed'
}
```

- nano/11.0.4
- Node.js/25.8.2
- CouchDb/2.3.1

Reproduce the changesReader example against CouchDB 2.3.1, then inspect node_modules/nano/lib/nano.js around responseHandler and the changesReader entry point. Done means the changes feed request is accepted by CouchDB rather than rejected with “Only GET allowed,” while preserving the shown feed options.

Written by the indexing model from the issue text.