[nexus] update transmit frames with IEEE 802.15.4 FCS (#12347)

This commit adds IEEE 802.15.4 FCS (Frame Check Sequence) computation
to the Nexus radio driver. Each frame to be transmitted is updated
with the proper CRC16-CCITT before being passed to the simulation.

Motivation for this change is to ensure that frames captured from
the Nexus simulation have a valid FCS, enabling proper decoding and
display in Wireshark when using pcap output.

tests/nexus/platform/nexus_radio.cpp

@@ -30,6 +30,7 @@
 
 #include "nexus_core.hpp"
 #include "nexus_node.hpp"
+#include "common/crc.hpp"
 
 namespace ot {
 namespace Nexus {
@@ -134,6 +135,9 @@ otError otPlatRadioTransmit(otInstance *aInstance, otRadioFrame *aFrame)
 
     VerifyOrExit(radio.mState == Radio::kStateReceive, error = kErrorInvalidState);
     OT_ASSERT(aFrame == &AsNode(aInstance).mRadio.mTxFrame);
+
+    static_cast<Radio::Frame *>(aFrame)->UpdateFcs();
+
     radio.mState = Radio::kStateTransmit;
 
     Core::Get().MarkPendingAction();
@@ -353,5 +357,19 @@ Radio::Frame::Frame(const Frame &aFrame)
     memcpy(mPsdu, aFrame.mPsdu, mLength);
 }
 
+void Radio::Frame::UpdateFcs(void)
+{
+    uint16_t fcs;
+
+    VerifyOrExit(mLength >= k154FcsSize);
+
+    fcs = CrcCalculator<uint16_t>(kCrc16CcittPolynomial).FeedBytes(mPsdu, mLength - k154FcsSize);
+
+    LittleEndian::WriteUint16(fcs, &mPsdu[mLength - k154FcsSize]);
+
+exit:
+    return;
+}
+
 } // namespace Nexus
 } // namespace ot

tests/nexus/platform/nexus_radio.hpp

@@ -55,6 +55,11 @@ struct Radio
         Frame(void);
         explicit Frame(const Frame &aFrame);
 
+        /**
+         * Updates the frame with the proper IEEE 802.15.4 FCS.
+         */
+        void UpdateFcs(void);
+
         uint8_t mPsduBuffer[kMaxFrameSize];
     };