feat(init): spawn service over IPC with per-client allowance refunded on child death; up to 4 death watchers; spawntest on test image

crates/kernel/src/syscall/ipc/events.rs

@@ -40,22 +40,27 @@ impl<P: Platform> Kernel<P> {
             _ => return Err(KernelError::WrongObject),
         };
         let target = self.target(pid, notification)?;
-        let slot = self
+        let watchers = self
             .death_watchers
             .get_mut(watched.raw() as usize)
             .ok_or(KernelError::InvalidProcess(watched))?;
+        let slot = watchers
+            .iter_mut()
+            .find(|w| w.is_none_or(|w| w == target))
+            .ok_or(KernelError::NoFreeSlot)?;
         *slot = Some(target);
         Ok(())
     }
 
-    /// A process is dying: tell its watcher, forget its own timer.
+    /// A process is dying: tell its watchers, forget its own timer.
     pub(crate) fn report_death(&mut self, pid: ProcessId) {
         self.timers.clear(pid);
-        if let Some(target) = self
+        let watchers = self
             .death_watchers
             .get_mut(pid.raw() as usize)
-            .and_then(Option::take)
-        {
+            .map(core::mem::take)
+            .unwrap_or_default();
+        for target in watchers.into_iter().flatten() {
             self.fire(target);
         }
     }