playlists: merge rules, sealing, and the key

playlist-merge.js and playlist-crypto.js have no imports and are run by
their tests, which is the only real evidence this feature can have.

Two revision counters per playlist, not one: a rename on one device and
a track added on another both write n+1, and a single counter makes two
edits that do not overlap collide.

One Argon2 run at sign-in, two handles. The AES handle is imported
non-extractably, so nothing can be derived from it — hence a second
import of the same bytes as HKDF rather than a derivation.

Measured: ~270 bytes a track, deflate worth 4.5x on realistic data, so
the 1 MB body cap holds about 17000 tracks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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