feat(kernel): add Kernel struct, MockPlatform, and simple syscalls (GetTime, GetProcessId, Yield)

crates/domain/src/scheduler/multi_core.rs

@@ -30,6 +30,10 @@ impl MultiCoreScheduler {
         }
     }
 
+    pub fn current(&self, core: CoreId) -> Option<ThreadId> {
+        self.cores[core.raw() as usize].current()
+    }
+
     fn thread_count_on(&self, core: CoreId) -> usize {
         self.placements
             .iter()