feat(approvals,audit): record the reviewer as the unchecked claim it is

The hub copies the reviewer string a client sends straight into
decided_by (DecideApproval, ClearEventLog); nothing on the wire ties
it to the authenticated caller. Studio filled it from the OS account,
so an export read like non-repudiation while being an arbitrary
client claim — the legal finding of the 2026-07-26 usertest panel.

The real fix is hub-side (derive decided_by from CALLER_IDENTITY);
that contract is written down in docs/reviewer-identity.md and needs
a hub release. Until then Studio does the one thing it can do
honestly and marks its own claim as a claim, inside the record:

- data/reviewer_identity.dart is the single place that produces and
  reads the value; wire() is idempotent, so page and HubService may
  both normalise. Every write path funnels through HubService, so no
  surface can send a bare handle.
- The inbox states before the decision who will be recorded, what
  that attribution is worth on this hub (from AuthStatus), and the
  literal string that lands in decided_by. An unreadable auth policy
  stays unreadable — never optimistic.
- Reading back: a marked value shows its plain name plus an
  unchecked flag; an unmarked one (legacy row, CLI decision, or a
  future hub-derived identity) is not classified either way.
- The audit wipe seeds the same kind of marked attribution into its
  chain.reset marker.

When the hub starts deriving the value it overwrites the field and
the prefix disappears by itself — no Studio release needed.

Guards: reviewer_identity_test (the value) and
approvals_reviewer_identity_test (every surface that writes or
renders it, against the hermetic fake hub). Visual proof for both
themes via the dialog-shot harness.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/support/fake_hub.dart

@@ -230,6 +230,55 @@ class FakeHubService extends Fake implements HubService {
   Future<List<Satellite>> listSatellites() =>
       _async('listSatellites', () => const []);
 
+  /// Auth policy the pages read (approvals derives from it what a
+  /// reviewer attribution is worth). Default: the local-dev hub —
+  /// static validator, anonymous calls allowed. Set to `null` to
+  /// script a hub whose policy Studio may not read; `authStatus`
+  /// then fails like a PermissionDenied.
+  HubAuthPolicy? authPolicy = const HubAuthPolicy(
+    validator: 'static',
+    anonymousAllowed: true,
+    tokens: [],
+  );
+
+  @override
+  Future<HubAuthPolicy> authStatus() => _async('authStatus', () {
+    final policy = authPolicy;
+    if (policy == null) {
+      throw const FakeGrpcError(7, 'admin scope required');
+    }
+    return policy;
+  });
+
+  /// Every decision that reached the wire, in order — the guard for
+  /// "Studio never claims an identity it cannot prove" inspects the
+  /// reviewer string exactly as the hub would store it.
+  final List<({String id, String reviewer, String? reason})> decisions = [];
+
+  /// Reviewer + reason handed to the audit-log wipe, same purpose.
+  final List<({String reviewer, String reason})> clears = [];
+
+  @override
+  Future<({int purged, String channel})> clearEventLog({
+    required String reviewer,
+    required String reason,
+  }) => _async('clearEventLog', () {
+    clears.add((reviewer: reviewer, reason: reason));
+    return (purged: 0, channel: 'local');
+  });
+
+  @override
+  Future<void> approve(String id, String reviewer) => _async(
+    'approve',
+    () => decisions.add((id: id, reviewer: reviewer, reason: null)),
+  );
+
+  @override
+  Future<void> reject(String id, String reviewer, String reason) => _async(
+    'reject',
+    () => decisions.add((id: id, reviewer: reviewer, reason: reason)),
+  );
+
   @override
   Future<SystemAiStatus> systemAiStatus() => _async(
     'systemAiStatus',