fix(node): a pinned device can be invited to a further invite-only group

Regression from device linking (Stage C, 2026-08-18). Once a device is
pinned on a node — as a member of one group, or an operator pairing — the
`known` fast-path in `_do_join_request` dropped straight into `_join_ok`.
For any *other* invite-only group it had no roster row for, that answered
`not_authorized_for_group` and stopped there: the client never got
`code_required`, so the pairing-code form never appeared and a legitimately
invited member could not join.

The `known` branch now, when there is no membership for the group being
opened:
  - with a valid code → consumes the invite and admits (as the unknown-
    device path already does);
  - with no code but an invite waiting for this user here → `code_required`,
    so the client prompts;
  - with no code and nothing inviting them → `not_authorized_for_group`,
    unchanged, so the H3 guarantee (a hub-invented pin gets no key) holds.

Also fixed: the group's own roster row is now consulted first, so an
existing member opening their group is never mistaken for a stranger.

Tests in test_roster_pairing.py cover all three branches plus the H3 guard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018gKJ85aZyvEwarXMFzFEwi

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