hub v0.60.0: offsite continuity Part B — superseded-escrow retention (data-first)

- host_escrow_superseded table + SaveHostEscrow retains a different-sha old blob before overwrite (tx); same-sha idempotent (no supersede row); returns superseded bool. ACK/restore read the current row unchanged. CountSuperseded/ListSuperseded; DeleteHost drops retained rows.
- escrow_superseded audit event + operator retained-count on host detail; register offbox_repo_orphaned/reset. Red-proof TestSaveHostEscrow_RetainsSuperseded.

hub/internal/web/templates/host_detail_body.html

@@ -263,7 +263,7 @@
                 </div>
                 <div class="info-item">
                     <span class="label">Key Escrow</span>
-                    <span class="value">{{if .EscrowPresent}}<span style="color: var(--green)">present</span>{{else}}<span class="text-muted">none</span>{{end}}</span>
+                    <span class="value">{{if .EscrowPresent}}<span style="color: var(--green)">present</span>{{else}}<span class="text-muted">none</span>{{end}}{{if gt .SupersededEscrowCount 0}} <span class="text-muted">· {{.SupersededEscrowCount}} superseded escrow blob(s) retained</span>{{end}}</span>
                 </div>
             </div>
         </section>