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

crates/domain/src/capability/mod.rs

@@ -17,7 +17,7 @@ pub struct Capability {
 }
 
 impl Capability {
-    pub(crate) fn new(
+    pub fn new(
         resource: Resource,
         permissions: PermissionSet,
         generation: Generation,

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()