feat: route bounded hierarchy postbox copies

docs/POSTBOX_CONCEPT.md

@@ -176,6 +176,24 @@ Vacancy is a visible delivery/attention state rather than an automatic grant
 to an unrelated personal account. Policy may trigger a bounded escalation
 after a delay.
 
+The implemented policy keeps the three semantics separate:
+
+- `linked_copy` can target the nearest matching ancestor or every bounded
+  matching ancestor in one selected structure.
+- `attention` currently supports delayed vacancy escalation over the remaining
+  delivery-time target snapshot.
+- `shared_visibility` remains explicitly disabled until its access and
+  encryption semantics are implemented.
+
+Routing is off unless an immutable template revision enables it and supplies a
+target template, target function type, producer allowlist, classification
+allowlist, depth, and structure. Optional relation, stop-unit, stop-unit-type,
+expiry, and maximum-retention gates narrow the route further. The dry-run API
+returns blocked and unavailable candidates without materializing addresses.
+Delivery materializes only frozen candidates, stores path-edge provenance, and
+creates source-preserving copies with independent read and acknowledgement
+receipts.
+
 ## Campaign Distribution
 
 Campaign can use Postbox as an explicit delivery channel through