R-1254 (ceremony half, HUNT-3 H2): after a failed escrow upload, ask the hub which package it holds before wording the result

The hub holds the new package -> success; another package -> 502 'previous code still works' (now true);
cannot be asked -> 504, no promise. Tests TestR1254_* red before the fix. Commit only; the agent release needs the vouch.

internal/localapi/escrow_ceremony_test.go

@@ -41,9 +41,13 @@ type fakeEscrowHub struct {
 	mu      sync.Mutex
 	current []byte
 	uploads int
-	err     error         // non-nil → the upload fails, nothing stored
-	gate    chan struct{} // non-nil → the upload blocks until closed
-	entered chan struct{} // non-nil → closed when an upload starts
+	err     error // non-nil → the upload fails, nothing stored
+	// R-1254 H2: lost → the upload STORES the package and then fails (the answer is lost on the way back);
+	// holdsErr → the hub cannot be asked which package it holds.
+	lost     bool
+	holdsErr error
+	gate     chan struct{} // non-nil → the upload blocks until closed
+	entered  chan struct{} // non-nil → closed when an upload starts
 }
 
 func (f *fakeEscrowHub) upload(ctx context.Context, pkg []byte) error {
@@ -61,9 +65,23 @@ func (f *fakeEscrowHub) upload(ctx context.Context, pkg []byte) error {
 	}
 	f.current = append([]byte(nil), pkg...)
 	f.uploads++
+	if f.lost {
+		return context.DeadlineExceeded
+	}
 	return nil
 }
 
+// holds answers „is the hub's current package exactly pkg?" — the R-1254 question (production compares the identity
+// blob the hub serves back).
+func (f *fakeEscrowHub) holds(_ context.Context, pkg []byte) (bool, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if f.holdsErr != nil {
+		return false, f.holdsErr
+	}
+	return string(f.current) == string(pkg), nil
+}
+
 // opensWith reports whether the hub's CURRENT package opens with code r — the customer's question.
 func (f *fakeEscrowHub) opensWith(r string) bool {
 	f.mu.Lock()
@@ -100,6 +118,7 @@ func newEscrowTestServerHub(t *testing.T, run ceremonyRunner, hub *fakeEscrowHub
 		HubConfigured: true,
 		DRConfigured:  func() bool { return true },
 		Upload:        hub.upload,
+		HubHolds:      hub.holds,
 	}
 	srv.ceremonyRun = run
 	srv.escrowSudoCheck = func(context.Context) error { return nil }