[nexus] implement microsecond alarm platform APIs (#12580)

This commit implements the otPlatAlarmMicro* platform APIs in the
Nexus simulation environment. It also transitions the simulation's
internal time base from millisecond to microsecond granularity to
ensure that both millisecond and microsecond alarms operate on a
consistent and shared time reference.

Changes include:
- Enabled OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE in Nexus config.
- Updated Nexus::Core to use a 64-bit microsecond time base (mNow).
- Implemented otPlatAlarmMicroGetNow, otPlatAlarmMicroStartAt, and
  otPlatAlarmMicroStop.
- Refactored Nexus::Node to support separate millisecond and
  microsecond alarm instances (mAlarmMilli and mAlarmMicro).
- Updated radio timestamps and PCAP logging to use the new high-
  resolution microsecond time base.
- Adjusted simulation time advancement and alarm triggering logic to
  handle both alarm types correctly.

tests/nexus/platform/nexus_node.cpp

@@ -38,7 +38,8 @@ void Node::Reset(void)
     uint32_t  id       = GetId();
 
     mRadio.Reset();
-    mAlarm.Reset();
+    mAlarmMilli.Reset();
+    mAlarmMicro.Reset();
     mMdns.Reset();
     mPendingTasklet = false;
 #if OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE