fix(protocol): harden gateway contract validation

frames/gateway-clipboard-v1.md

@@ -10,8 +10,8 @@ flow. Its UTF-8 JSON payload is a `GatewayClipboardText` object:
 `direction` is exact: the client may send only `client_to_provider`, and the
 gateway may send only `provider_to_client`. The text contains no file name,
 URL, binary value, or client-folder field and is at most the Server-owned
-`clipboard_policy.max_text_bytes` value. `loop_token` is a 16--128 ASCII
-base64url-character token generated by the originating endpoint. An endpoint MUST retain
+`clipboard_policy.max_text_bytes` value. `loop_token` is a 16--128 character
+canonical unpadded ASCII base64url token generated by the originating endpoint. An endpoint MUST retain
 recent token/value pairs only for the bounded policy window and MUST suppress a
 matching reflected value; a mismatched, malformed, expired, or replayed token
 is rejected without clipboard mutation.
@@ -21,8 +21,8 @@ direction, a rate above `max_updates_per_minute`, invalid UTF-8, an oversized
 payload, or an unknown field fails closed. Clipboard bytes are never emitted to
 provider-state, audit, telemetry, or error payloads.
 
-For every accepted, loop-suppressed, or policy/rate/provider/malformed rejection,
-the gateway sends an mTLS control-plane `GatewayClipboardAudit` record. It contains
+For every successfully delivered, loop-suppressed, or policy/rate/provider/malformed
+rejection, the gateway sends an mTLS control-plane `GatewayClipboardAudit` record. It contains
 only the session identifier, direction, bounded text-byte count, outcome, and a
 fixed reason code; it contains neither text nor loop token. The Server persists it
 against the broker session using the authenticated gateway identity.