fix: an MBK2 bundle is opened once and stored again as MBK3

Transitional. The Argon2 run that makes M makes A, the key MBK2 bundles were
sealed under; a session keeps it as a decrypt-only key (IndexedDB in a browser,
the key storage in the desktop app). A client meeting an MBK2 bundle opens it —
or its recovery copy — and stores the same identity as MBK3 once connected; the
desktop app reseals or withdraws it as browser access says. A session without
A asks for the passphrase once. Older formats stay refused by name. Replaces
the unpin-and-reinvite step the 0.17 flag day required on every node.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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