Shared mailboxes: a second kind of account lock

A shared mailbox (support@, legal@) belongs to no one person: nobody
signs in to it, and the people assigned open it beside their own mail
at an access level an administrator chose. An account lock already is
most of that: it keeps receiving mail, refuses every sign-in, and its
delegates reach it through real grants on every container (so IMAP,
DAV and JMAP honor them), never including Share. So a shared mailbox is
a lock of a second kind (multi-account spec, MA-S; John, 2026-10-05).

Lock gains kind: "lock" (the default, so stored locks read as before)
or "sharedMailbox", set on create and fixed after. A shared mailbox:

- needs no reason to make, change or end;
- holds up to 100 people, where a lock holds 10;
- runs its own Sieve replies and redirects, so an automatic
  acknowledgement goes out (a lock answers no one);
- records only what is sent as it (audit_send_as, which now covers it),
  not AL-9's access and per-change records, which would bury the log
  for a busy desk;
- sends only as itself (MA-S3): From and Reply-To must be its own
  addresses, so answers come back to the mailbox and not to whoever
  replied; anything else is forbiddenFrom.

The session marks it delegation: {locked: true, kind: "sharedMailbox"},
so a front end that knows no kind still treats it as a lock. The
console's layout gains Management › Directory › Shared Mailboxes
(CustomComponent/SharedMailboxes).

Tests: the account lock suite now goes on to a shared mailbox: made
without a reason with twelve people, sign-in refused, the session's
kind, its vacation reply delivered, an answer sent as it and recorded
as the agent with no per-change records, and a Reply-To naming the
agent refused; a lock unit test reads a stored lock without a kind.
account_lock_tests, jmap_tests, audit_log_tests and imap_tests pass
(RocksDB).

crates/common/src/auth/mod.rs

@@ -152,6 +152,8 @@ pub struct AccessTokenInner {
     pub(crate) obj_size: u64,
     // inbuxa: AL-2: the account is locked; it may not authenticate
     pub(crate) locked: bool,
+    // inbuxa: MA-S: the lock is a shared mailbox
+    pub(crate) shared_mailbox: bool,
     // inbuxa: AL-5: locked accounts handed to this one
     pub(crate) delegations: Box<[Delegation]>,
 }
@@ -165,6 +167,8 @@ pub struct Delegation {
     pub send_as: bool,
     /// Seconds since the epoch.
     pub until: Option<u64>,
+    /// MA-S: a locked account, or a shared mailbox.
+    pub kind: inbuxa_features::lock::Kind,
 }
 
 #[derive(Debug, Default, Hash, Clone)]