feat: implement context switching, SavedContext with pc/status fields, wire all syscall outcomes and timer preemption

crates/kernel/src/tests/helpers.rs

@@ -97,8 +97,4 @@ impl Kernel<MockPlatform> {
     ) -> CapSlot {
         self.grant_cap(pid, resource, permissions).expect("grant cap")
     }
-
-    pub fn thread_context(&self, tid: ThreadId) -> &domain::thread::SavedContext {
-        self.threads.get(tid).expect("thread").saved_context()
-    }
 }

crates/kernel/src/tests/lifecycle.rs

@@ -28,9 +28,9 @@ fn spawn_thread_preserves_entry_and_stack() {
         SyscallOutcome::Complete(SyscallResult::Value(v)) => ThreadId::from_raw(v as u16),
         _ => panic!("expected Value"),
     };
-    let ctx = k.thread_context(tid);
+    let ctx = k.thread_context(tid).unwrap();
     assert_eq!(ctx.program_counter(), 0xDEAD);
-    assert_eq!(ctx.stack_pointer(), 0xBEEF);
+    assert_eq!(ctx.registers()[2], 0xBEEF);
 }
 
 #[test]