Mirrors the iOS side. Create mints a gid, group key, owner signing key and
one random tag per member, seals the roster and hands the relay a blob —
no member id leaves the device. Fetch asks by TAG, never "what groups am I
in", because a sealed relay cannot answer that.
Reachability moved here from the relay, which needed to know who the
members were in order to check it.
⚠️ An invitation is NOT subject to the roster check the group name goes
through, and cannot be: that check asks the relay who is in the group, and
for a group we have not joined yet we cannot fetch it to ask. What guards
it instead is that an invitation is accepted only for a group we hold no
secrets for — a stranger cannot overwrite a group we are really in, which
would make every later roster fail to open and silently empty the
conversation.
The base64 stays inside the engine: `Bytes` is internal there on purpose,
so `SealedRoster` grew `newGroupId`, `newOwnerKey`, `ownerPublic` and
`signRoster` rather than the app reaching past that boundary.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
