Resolve v0.16 domain ids to names before comparing directories

x:Account.domainId is an internal id on v0.16 ("b"), not a domain name. A
pre-migration snapshot taken from a v0.15 instance records names
("smoke.test"), so the post-migration directory comparison compared ids
against names and would have reported every domain as having vanished -
a false alarm on the check whose whole job is proving nothing was lost.

The client now resolves them with x:Domain/query + x:Domain/get in a single
request, using a JMAP back-reference (RFC 8620 3.7). Confirmed against a
live 0.16.14 before being written:

    ["x:Domain/get", {"list":[{"name":"smoke.test","id":"b"}]}, "g"]

An id that can't be resolved is kept as-is - a domain that can't be named is
still a domain that exists - but a failure of the resolution call itself is
an error rather than a silent fallback, since quietly comparing ids against
names is precisely the bug being fixed.

Verified against the live migrated instance: the snapshot that reported
domains=[b] now reports domains=[smoke.test], matching what the
pre-migration snapshot recorded.

ARCHITECTURE.md

@@ -715,12 +715,16 @@ happens to need them. `preflight.DeploymentKind` is a type alias for
   far larger blast radius for no benefit. Where v0.15 listed several roles,
   admin wins and the collapse is reported; roles with no v0.16 equivalent
   are named rather than dropped silently.
-- **`x:Account.domainId` returns an internal id on v0.16, not a domain
-  name.** A pre-migration snapshot records domains as names
-  ("smoke.test"); the same instance after migration reports "b". The
-  directory comparison in §4.7 would read that as every domain having
-  vanished. Resolving ids to names needs an `x:Domain/get` call that hasn't
-  been confirmed against the binary yet.
+- **`x:Account.domainId` returns an internal id on v0.16 - resolved.** A
+  pre-migration snapshot records domains as names ("smoke.test"); the same
+  instance afterwards reported "b", so the §4.7 directory comparison would
+  have read every domain as having vanished. The client now resolves ids to
+  names with `x:Domain/query` + `x:Domain/get` in a single request via a
+  JMAP back-reference, confirmed against a live 0.16.14. An id that cannot
+  be resolved is kept as-is rather than dropped, since a domain that can't
+  be named is still a domain that exists; a failure of the resolution call
+  itself is an error, because silently comparing ids against names is the
+  bug this fixes.
 - **Quota recalculation is grounded but unproven.** The `x:Task` wire
   format comes from Stalwart's schema reference rather than a live server;
   §4.5 lists exactly which two details are inferred. A smoke test against a