NRM: P/F burst fix, CR bit fix, disconnected state filter, 9 new tests

Library fixes:
- P/F bit now only set on last I-frame in NRM burst (enables full window
  throughput). S/U frames always get P/F for marker passing.
- Add missing CR bit on idle-path SNRM (consistency with timeout path)
- Add disconnected state filter in __on_u_frame_read(): rejects non-SABM/
  SNRM/UI U-frames when peer is disconnected, sends DM for commands
- Update ABM test to expect DM (not UA) for DISC when disconnected

New NRM tests (9 added, 82 total):
- PrimaryToSecondary_SingleIFrame: I-frame with P bit on single frame
- MultiplIFrames_PFOnLastOnly: P/F burst - first frame no P, last has P
- SecondaryToRrimary_IFrameReceive: data reception + RR acknowledgment
- MarkerTimeoutRecovery: primary reclaims marker after retry_timeout
- PrimaryDisconnect: DISC+P / UA+F exchange
- DMResponseWhenDisconnected: DM+F for command when disconnected
- RoundRobinPeerSwitching: alternating I-frames between 2 peers
- FRMRRecovery: FRMR triggers disconnect + SNRM re-establishment
- REJRecovery: REJ retransmission with proper round-robin cycle

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lexus2k

src/proto/fd/tiny_fd.c

@@ -531,7 +531,7 @@ static uint8_t *tiny_fd_get_next_frame_to_send(tiny_fd_handle_t handle, int *len
             ( handle->peers[peer].state == TINY_FD_STATE_DISCONNECTED || handle->peers[peer].state == TINY_FD_STATE_CONNECTING))
         {
             tiny_frame_header_t frame = {
-                .address = address,
+                .address = address | HDLC_CR_BIT,
                 .control = HDLC_U_FRAME_TYPE_SNRM | HDLC_U_FRAME_BITS,
             };
             __put_u_s_frame_to_tx_queue(handle, TINY_FD_QUEUE_S_FRAME, &frame, 2);
@@ -552,8 +552,19 @@ static uint8_t *tiny_fd_get_next_frame_to_send(tiny_fd_handle_t handle, int *len
         tiny_frame_header_t *header = (tiny_frame_header_t *)data;
         if ( handle->mode == TINY_FD_MODE_NRM )
         {
-            // NRM: Always set P/F bit for marker passing
-            header->control |= HDLC_P_BIT;
+            // NRM: P/F on last frame only. S/U frames always get P/F (marker passing).
+            // I-frames get P/F only when no more I-frames remain for this peer.
+            if ( (header->control & HDLC_I_FRAME_MASK) == HDLC_I_FRAME_BITS )
+            {
+                if ( __all_frames_are_sent(&handle->peers[peer].i_queue_control) )
+                {
+                    header->control |= HDLC_P_BIT;
+                }
+            }
+            else
+            {
+                header->control |= HDLC_P_BIT;
+            }
         }
         else
         {

src/proto/fd/tiny_fd_on_rx_int.h

@@ -174,6 +174,22 @@ static int __on_u_frame_read(tiny_fd_handle_t handle, uint8_t peer, void *data,
     uint8_t type = control & HDLC_U_FRAME_TYPE_MASK;
     int result = TINY_ERR_FAILED;
     LOG(TINY_LOG_INFO, "[%p] Receiving U-Frame type=%02X with address [%02X]\n", handle, type, ((uint8_t *)data)[0]);
+    // In disconnected state, only accept connection setup (SABM/SNRM) and connectionless (UI) frames.
+    // Respond with DM to commands; silently ignore responses.
+    if ( handle->peers[peer].state == TINY_FD_STATE_DISCONNECTED &&
+         type != HDLC_U_FRAME_TYPE_SABM && type != HDLC_U_FRAME_TYPE_SNRM && type != HDLC_U_FRAME_TYPE_UI )
+    {
+        LOG(TINY_LOG_WRN, "[%p] Ignoring U-frame type=%02X in disconnected state\n", handle, type);
+        if ( ((uint8_t *)data)[0] & HDLC_CR_BIT )
+        {
+            tiny_frame_header_t frame = {
+                .address = __peer_to_address_field( handle, peer ),
+                .control = HDLC_U_FRAME_TYPE_DM | HDLC_U_FRAME_BITS,
+            };
+            __put_u_s_frame_to_tx_queue(handle, TINY_FD_QUEUE_U_FRAME, &frame, 2);
+        }
+        return result;
+    }
     if ( type == HDLC_U_FRAME_TYPE_SABM || type == HDLC_U_FRAME_TYPE_SNRM )
     {
         tiny_frame_header_t frame = {

unittest/tiny_fd_abm_tests.cpp

@@ -181,11 +181,10 @@ TEST(TINY_FD_ABM, ABM_DisconnectResponseWhenNotConnected)
     CHECK_EQUAL(TINY_SUCCESS, read_result);
     int len = tiny_fd_get_tx_data(handle, outBuffer.data(), outBuffer.size(), 100);
     CHECK_EQUAL(4, len);
-    // Check UA frame   
-    // UA frame should be: 0x7E 0x03 0x00 0x7E
+    // Per HDLC spec: DISC when disconnected gets DM response (not UA)
     CHECK_EQUAL(0x7E, outBuffer[0]); // Flag
     CHECK_EQUAL(0x01, outBuffer[1]); // address field - CR bit must be cleared
-    CHECK_EQUAL(0x73, outBuffer[2]); // UA packet
+    CHECK_EQUAL(0x1F, outBuffer[2]); // DM packet with P/F bit
     CHECK_EQUAL(0x7E, outBuffer[3]); // Flag
     CHECK(connected); // Connection should not be changed
 }