fix(node): device messages are authenticated-only, and the code now says so

`device_add_request` and `device_hello` were dispatched behind
`and self._nonce_node`, which reads as "pre-proof, once the challenge has gone
out" — and is not what happens: both branches sit after the
`self._user_id is None` guard, so the nonce is always set by the time either is
reached, and a peer that has not finished its handshake gets "Handshake
required" instead.

The guard is removed rather than the branches moved. Filing a device is not
something a peer needs *in order to* prove possession of the group key, which is
the only reason anything is served pre-proof: the request is countersigned later
by a device already pinned, so requiring the caller to finish its own handshake
first costs nothing and keeps the pre-proof surface at three messages.

A test drives all six device messages through the real dispatcher on an
unauthenticated session, because this is a property of the order of its branches
and of nothing else.

Node suite 1216 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

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