fix(hub): the password verifier is Argon2id 64 MiB, and a hash's version names its parameters

`pw_version` 4: Argon2id 64 MiB, t=3, lanes=4 — RFC 9106's second recommended
setting. A v3 hash (256 MB) still verifies at its own parameters and is
rewritten at the new ones on the next sign-in, through the rehash path that
already existed.

Why not more. The verifier matters against an offline attacker holding the
database; online guessing is bounded by the sign-in lockout. That attacker pays
the client's 600 000 PBKDF2-SHA512 iterations and the hub's Argon2id per guess,
since `auth_key` is 256 bits and cannot be searched directly. Memory above
64 MiB multiplies that cost by a constant — at most 16 at 256 MB, less with
PBKDF2 counted — while the hub pays the same memory at every sign-in, one
derivation at a time. Measured on meshbay.org: 450 ms at 256 MB, 105 ms at
64 MiB, so a burst of sign-ins clears about four times faster.

Changing the current version exposed a latent lockout. `hash_password` always
used the current version's parameters, while the raw-password scheme recorded
`pw_version = 2` — harmless while versions 2 and 3 shared their parameters,
and with version 4 every legacy registration and v1→v2 rehash would have
stored a 64 MiB hash labelled 256 MB, which nothing could then verify. Seventeen
tests caught it. `hash_password` now takes the version it is hashing for.

The OpenSSL deadlock between two concurrent `lanes=4` derivations is the same at
64 MiB, so Argon2 stays on its single worker. The loop-stall test measures
against a v3 hash, because half of a 45 ms inline derivation is too close to
scheduling noise to be a reliable bound.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LcF3QKWii7uQ2kSyXErzCt

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