feat: replace private cas envelope

docs/command-stability.md

@@ -57,22 +57,17 @@ The following commands are tested but may still change while the subsystem
 settles:
 
 - `geth overlay status|plan|join|leave|interface-plan|up|down|peers|send|recv`
+- `geth cas add-private`
+- `geth cas get-private`
 
 Migration expectation: overlay membership records and authorization resources
 should remain readable, but packet runtime flags, platform activation details,
 and route metadata may change before the first deployment tag.
-
-## Prototype
-
-The following commands intentionally do not claim a stable security or storage
-contract yet:
-
-- `geth cas add-private`
-- `geth cas get-private`
-
-Migration expectation: prototype private CAS envelopes may be replaced by an
-audited AEAD/key-envelope design. Existing prototype blobs may need an explicit
-re-encryption or export/import workflow.
+Private CAS writes use an AES-256-GCM envelope, but the command family remains
+experimental while key envelopes, remote sharing, and forward-secrecy/PCS
+properties are explicitly out of scope. Prototype BLAKE3-XOR envelopes from
+earlier pre-deployment builds are rejected with a clear error and should be
+recreated from plaintext.
 
 ## Adding Commands