Address code review feedback: fix bitfield, frame_length, add thread-safety note

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

Author: Copilot

include/sdlp_tc.h

@@ -16,7 +16,7 @@ typedef struct {
     uint8_t virtual_channel_id : 6;
     uint8_t reserved2 : 2;
     uint16_t frame_length : 10;
-    uint16_t frame_sequence_number : 8;
+    uint8_t frame_sequence_number;
 } sdlp_tc_header_t;
 
 typedef struct {

src/sdlp_tc.c

@@ -17,7 +17,7 @@ int sdlp_tc_create_frame(sdlp_tc_frame_t *frame, uint16_t spacecraft_id,
     frame->header.spacecraft_id = spacecraft_id & 0x3FF;
     frame->header.virtual_channel_id = virtual_channel_id & 0x3F;
     frame->header.reserved2 = 0;
-    frame->header.frame_length = data_length + TC_PRIMARY_HEADER_SIZE - 1;
+    frame->header.frame_length = data_length - 1;
     frame->header.frame_sequence_number = frame_seq_num;
 
     memcpy(frame->data, data, data_length);

src/sdlp_tm.c

@@ -1,6 +1,7 @@
 #include "sdlp_tm.h"
 #include <string.h>
 
+/* Note: not thread-safe, caller must ensure sequential access */
 static uint8_t tm_frame_counter = 0;
 
 int sdlp_tm_create_frame(sdlp_tm_frame_t *frame, uint16_t spacecraft_id,