fix: Align channel data framing

ref: #1928

docs/companion_protocol.md

@@ -303,7 +303,7 @@ Bytes 7+: Binary payload bytes (variable length)
 - Values other than `0xFF` are reserved for official protocol extensions.
 
 **Limits**:
-- Maximum payload length is `163` bytes (`MAX_GROUP_DATA_LENGTH`).
+- Maximum payload length is `160` bytes.
 - Larger payloads are rejected with `PACKET_ERROR` / `ERR_CODE_ILLEGAL_ARG`.
 
 **Response**: `PACKET_OK` (0x00) on success
@@ -326,7 +326,7 @@ Byte 0: 0x0A
 
 **Response**: 
 - `PACKET_CHANNEL_MSG_RECV` (0x08) or `PACKET_CHANNEL_MSG_RECV_V3` (0x11) for channel messages
-- `PACKET_CHANNEL_DATA_RECV` (0x1B) or `PACKET_CHANNEL_DATA_RECV_V3` (0x1C) for channel data
+- `PACKET_CHANNEL_DATA_RECV` (0x1B) for channel data
 - `PACKET_CONTACT_MSG_RECV` (0x07) or `PACKET_CONTACT_MSG_RECV_V3` (0x10) for contact messages
 - `PACKET_NO_MORE_MSGS` (0x0A) if no messages available
 
@@ -397,8 +397,7 @@ Messages are received via the TX characteristic (notifications). The device send
    - `PACKET_CHANNEL_MSG_RECV_V3` (0x11) - Version 3 with SNR
 
 2. **Channel Data**:
-   - `PACKET_CHANNEL_DATA_RECV` (0x1B) - Standard format
-   - `PACKET_CHANNEL_DATA_RECV_V3` (0x1C) - Version 3 with SNR
+   - `PACKET_CHANNEL_DATA_RECV` (0x1B) - Includes SNR and reserved bytes
 
 3. **Contact Messages**:
    - `PACKET_CONTACT_MSG_RECV` (0x07) - Standard format
@@ -502,26 +501,17 @@ Bytes 11+: Payload bytes
 
 ### Channel Data Format
 
-**Standard Format** (`PACKET_CHANNEL_DATA_RECV`, 0x1B):
+**Format** (`PACKET_CHANNEL_DATA_RECV`, 0x1B):
 ```
 Byte 0: 0x1B (packet type)
-Byte 1: Channel Index (0-7)
-Byte 2: Path Length
-Byte 3: Data Type
-Bytes 4-7: Timestamp (32-bit little-endian)
-Bytes 8+: Payload bytes
-```
-
-**V3 Format** (`PACKET_CHANNEL_DATA_RECV_V3`, 0x1C):
-```
-Byte 0: 0x1C (packet type)
 Byte 1: SNR (signed byte, multiplied by 4)
 Bytes 2-3: Reserved
 Byte 4: Channel Index (0-7)
 Byte 5: Path Length
 Byte 6: Data Type
-Bytes 7-10: Timestamp (32-bit little-endian)
-Bytes 11+: Payload bytes
+Byte 7: Data Length
+Bytes 8-11: Timestamp (32-bit little-endian)
+Bytes 12+: Payload bytes
 ```
 
 **Parsing Pseudocode**:
@@ -529,9 +519,10 @@ Bytes 11+: Payload bytes
 def parse_channel_frame(data):
     packet_type = data[0]
     offset = 1
+    snr = None
     
-    # Check for V3 format
-    if packet_type in (0x11, 0x1C):  # V3
+    # Formats with explicit SNR/reserved bytes
+    if packet_type in (0x11, 0x1B):
         snr_byte = data[offset]
         snr = ((snr_byte if snr_byte < 128 else snr_byte - 256) / 4.0)
         offset += 3  # Skip SNR + reserved
@@ -539,8 +530,10 @@ def parse_channel_frame(data):
     channel_idx = data[offset]
     path_len = data[offset + 1]
     item_type = data[offset + 2]
-    timestamp = int.from_bytes(data[offset+3:offset+7], 'little')
-    payload = data[offset+7:]
+    data_len = data[offset + 3] if packet_type == 0x1B else None
+    timestamp = int.from_bytes(data[offset+4:offset+8], 'little') if packet_type == 0x1B else int.from_bytes(data[offset+3:offset+7], 'little')
+    payload_offset = offset + 8 if packet_type == 0x1B else offset + 7
+    payload = data[payload_offset:payload_offset + data_len] if packet_type == 0x1B else data[payload_offset:]
     is_text = packet_type in (0x08, 0x11)
     if is_text and item_type == 0:
         message = payload.decode('utf-8')
@@ -553,7 +546,7 @@ def parse_channel_frame(data):
         'timestamp': timestamp,
         'payload': payload,
         'message': message,
-        'snr': snr if packet_type in (0x11, 0x1C) else None
+        'snr': snr
     }
 ```
 
@@ -590,8 +583,7 @@ Use `CMD_SEND_CHANNEL_TXT_MSG` for plain text, and `CMD_SEND_CHANNEL_DATA` for b
 | 0x10  | PACKET_CONTACT_MSG_RECV_V3 | Contact message (V3 with SNR) |
 | 0x11  | PACKET_CHANNEL_MSG_RECV_V3 | Channel message (V3 with SNR) |
 | 0x12  | PACKET_CHANNEL_INFO        | Channel information           |
-| 0x1B  | PACKET_CHANNEL_DATA_RECV   | Channel data (standard)       |
-| 0x1C  | PACKET_CHANNEL_DATA_RECV_V3| Channel data (V3 with SNR)    |
+| 0x1B  | PACKET_CHANNEL_DATA_RECV   | Channel data (includes SNR)   |
 | 0x80  | PACKET_ADVERTISEMENT       | Advertisement packet          |
 | 0x82  | PACKET_ACK                 | Acknowledgment                |
 | 0x83  | PACKET_MESSAGES_WAITING    | Messages waiting notification |
@@ -892,7 +884,7 @@ def on_notification_received(data):
     packet_type = data[0]
     
     if packet_type in (PACKET_CHANNEL_MSG_RECV, PACKET_CHANNEL_MSG_RECV_V3,
-                       PACKET_CHANNEL_DATA_RECV, PACKET_CHANNEL_DATA_RECV_V3):
+                       PACKET_CHANNEL_DATA_RECV):
         message = parse_channel_frame(data)
         handle_channel_message(message)
     elif packet_type == PACKET_MESSAGES_WAITING: