Who may share mail: a server switch, and a tenant's that can only be stricter

A school, or any organization that doesn't want people's mailboxes
shared, can now turn that off (multi-account spec, MA-C). Two switches
at two levels, as the legacy-protocols switch has:

- mailSharing: people may share their own mail folders;
- addAccounts: people may add other accounts to the webmail (read by
  the webmail's account switcher, MA-B).

inbuxa:SharingPolicy/get and /set hold them: the server's policy has
the singleton id, each tenant's has the tenant's id. Both default to
on, so nothing changes until someone turns one off. A tenant's
administrator changes their own tenant's (the domain's permissions, as
for its protocols switch); only a server administrator with
sysSharingUpdate changes the server's; a tenant can never be looser
than the server (forbidden). Every change goes through the audit log,
and rebuilds every access token, here and on every node.

With mail sharing off for an account's tenant (or the server):

- Mailbox/set and IMAP SETACL refuse to start or widen a share
  (forbidden / NO [NOPERM]); narrowing or ending one is always allowed;
- shares already made give nothing while it is off: an access token
  leaves out mailbox grants from such an owner. They stay stored, so
  turning sharing back on restores them (John, 2026-10-05);
- a lock's and a shared mailbox's grants are an administrator's and
  always count, and group membership was never a share.

The session's own account says mailSharing and addAccounts, the
stricter of the two levels, so front ends can hide what is off.

Tests: a new sharing_policy suite with a school tenant, its own
administrator and two people outside it: on by default; the school's
administrator turns it off but can't touch the server's; an old share
stops working and a new one is refused while someone outside the school
is unaffected; a shared mailbox in the school keeps working; the server
off can't be loosened by the tenant; on again restores the old share;
ending a share works while off; and every change is audited. A unit
test covers the stricter-only rule. sharing_policy_tests, jmap_tests,
imap_tests, account_lock_tests and audit_log_tests pass (RocksDB).

crates/jmap-proto/src/references/resolve.rs

@@ -64,6 +64,9 @@ impl Response<'_> {
                 GetRequestMethod::TenantProtocolPolicy(request) => {
                     request.resolve_references(self)?
                 }
+                GetRequestMethod::SharingPolicy(request) => {
+                    request.resolve_references(self)?
+                }
                 GetRequestMethod::Principal(request) => request.resolve_references(self)?,
                 GetRequestMethod::Quota(request) => request.resolve_references(self)?,
                 GetRequestMethod::Blob(request) => request.resolve_references(self)?,
@@ -158,6 +161,9 @@ impl Response<'_> {
                 SetRequestMethod::TenantProtocolPolicy(request) => {
                     request.resolve_references(self, 1, false)?
                 }
+                SetRequestMethod::SharingPolicy(request) => {
+                    request.resolve_references(self, 1, false)?
+                }
                 SetRequestMethod::AddressBook(request) => {
                     request.resolve_references(self, 1, false)?
                 }