fix(protocol): align terminal receipt conformance

frames/gateway-input-feedback-v1.md

@@ -53,8 +53,9 @@ is the following exact envelope:
 | 6 | 2 | payload length | exact payload byte count |
 | 8 | N | payload | exact type-specific body |
 
-The client-to-gateway types are `0x01` IDR request (empty) and `0x02` FEC
-status: `frame_index` (u32), `highest_received_sequence` (u16),
+The client-to-gateway types are `0x01` IDR request (empty), `0x02` FEC
+status, and `0x03` terminal receipt (empty). FEC status contains
+`frame_index` (u32), `highest_received_sequence` (u16),
 `next_contiguous_sequence` (u16), `missing_before_highest` (u16),
 `total_data_packets` (u16), `total_parity_packets` (u16),
 `received_data_packets` (u16), `received_parity_packets` (u16),
@@ -63,6 +64,10 @@ status: `frame_index` (u32), `highest_received_sequence` (u16),
 the provider's unsequenced ENet FEC delivery; it does not put it on the reliable
 provider input path.
 
+The terminal receipt is valid only from client to gateway with an exact
+zero-byte payload. Session-state authorization remains a gateway responsibility;
+the Protocol grammar defines only its fixed wire shape.
+
 The gateway-to-client types are `0x10` host termination (`exit_code` u32),
 `0x11` rumble (`controller` u8, `low_frequency` u16,
 `high_frequency` u16), and `0x12` HDR mode (`enabled` exactly `0` or `1`). The