docs(core): refine function-bound postbox encryption

docs/POSTBOX_E2EE_ARCHITECTURE.md

@@ -57,6 +57,11 @@ The trust layer should provide:
 - key rotation and epoch tracking
 - recovery policy hooks
 
+Recovery must be organizationally governed. A server-held universal plaintext
+key would defeat the E2EE claim; any escrow, threshold recovery, or emergency
+grant needs an explicit assurance profile, authority/quorum, audit trail, and
+user-visible consequence.
+
 ## Role And Function Postboxes
 
 Role-bound access needs special handling. A postbox can be bound to an
@@ -75,6 +80,30 @@ rewrapping service:
 Key epochs are required when role membership changes. Older messages may remain
 readable according to policy, but new access must use the current epoch.
 
+The function-bound container exists independently of membership. It may remain
+vacant and continue to receive ciphertext without falling back to an unrelated
+personal mailbox. Zero, one, or several incumbents are valid states. Each
+incumbent receives an independently auditable, device-bound wrapped-key path;
+the postbox is never copied into their account ownership.
+
+A new assignment or hand-over rotates the function/postbox key epoch. Envelope
+encryption permits the normal rotation path to rewrap per-message data keys
+rather than rewrite large ciphertext objects; a security policy may require
+full content re-encryption for selected compromise or cryptographic-profile
+events. The history available to a new incumbent must be selected policy (all
+retained history, a bounded historical window, or assignment-time content) and
+recorded with the grant.
+
+Delegation is a time-bounded represented-function grant, not a copy or
+substitution of the postbox. Expiry or withdrawal stops future key release and
+actions. It cannot revoke plaintext already decrypted, printed, exported, or
+captured outside the platform. Multiple simultaneous incumbents and delegates
+remain distinguishable in key-fetch and action evidence.
+
+Postbox content and signed manifests are immutable. Correction or replacement
+creates a linked new object/version; it never silently substitutes ciphertext
+or evidence that another actor may already have inspected.
+
 ## External Recipients
 
 External recipients may need one-time or time-limited access without a full