Ensure reserved bits are zero-filled in TC encoding

Co-authored-by: Michal-Pogorzelec <80826463+Michal-Pogorzelec@users.noreply.github.com>

Author: Copilot

src/sdlp_tc.c

@@ -46,7 +46,7 @@ int sdlp_tc_encode_frame(const sdlp_tc_frame_t *frame, uint8_t *buffer,
                        ((frame->header.reserved & 0x03) << 2) |
                        ((frame->header.spacecraft_id >> 8) & 0x03);
     buffer[offset++] = frame->header.spacecraft_id & 0xFF;
-    buffer[offset++] = (frame->header.virtual_channel_id & 0x3F) << 2;
+    buffer[offset++] = ((frame->header.virtual_channel_id & 0x3F) << 2) | 0x00;
     buffer[offset++] = (frame->header.frame_length >> 8) & 0xFF;
     buffer[offset++] = frame->header.frame_length & 0xFF;