[nexus] implement CSL transmitter and receiver (#12595)

This commit implements CSL transmitter and receiver functionality in
the Nexus simulation platform. It enables the necessary OpenThread
configurations and provides the platform-level support for CSL.

It also implements the otPlatRadioGetNow platform API to provide a
high-resolution 64-bit microsecond time base, which is required for
accurate CSL timing and synchronization.

Changes:
- Added GetNowMicro64() to Nexus::Core to expose the raw 64-bit
  microsecond timer.
- Implemented otPlatRadioGetNow() in nexus_radio.cpp.
- Enabled OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE and
  OPENTHREAD_CONFIG_MAC_CSL_TRANSMITTER_ENABLE in Nexus config.
- Added otRadioContext to Nexus::Radio to track CSL parameters and
  manage security material for radio operations.
- Implemented otPlatRadioEnableCsl, otPlatRadioResetCsl,
  otPlatRadioUpdateCslSampleTime, and otPlatRadioGetCslAccuracy.
- Implemented otPlatRadioSetMacKey, otPlatRadioSetMacFrameCounter,
  and otPlatRadioSetAlternateShortAddress to keep radio context
  synchronized with OpenThread stack.
- Updated otPlatRadioSetExtendedAddress and otPlatRadioEnableCsl to
  use AsCoreType for better readability and project conventions.
- Moved Radio member initializations to the initializer list in its
  constructor.
- Refactored radio SFD processing: moved otMacFrameProcessTxSfd and
  UpdateFcs from otPlatRadioTransmit (radio platform) to
  Core::ProcessRadio (simulation engine), ensuring CSL IEs and
  security headers are updated exactly when transmission starts.
- Enhanced Nexus::Core::ProcessRadio to support generating Enhanced
  ACKs with CSL IEs for 802.15.4-2015 frames.
- Updated Nexus and fuzz build systems to include necessary Nexus
  platform and utils headers/utilities.

tests/fuzz/CMakeLists.txt

@@ -31,6 +31,8 @@ set(COMMON_INCLUDES
     ${PROJECT_SOURCE_DIR}/src
     ${PROJECT_SOURCE_DIR}/src/core
     ${PROJECT_SOURCE_DIR}/tests/nexus
+    ${PROJECT_SOURCE_DIR}/tests/nexus/platform
+    ${PROJECT_SOURCE_DIR}/examples/platforms/utils
 )
 
 set(COMMON_COMPILE_OPTIONS