feat: add SavedContext::with_entry_and_stack, wire SpawnThread to preserve entry point and stack

crates/kernel/src/tests/helpers.rs

@@ -97,4 +97,8 @@ 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()
+    }
 }