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

scripts/make-disk-image.sh

@@ -33,6 +33,8 @@ copy session-server "session.elf"
 # Only the test image: its presence makes init run the SMP self-test.
 if [ -n "${SMPTEST:-}" ]; then
     copy smptest    "smptest.elf"
+    copy spawntest  "spawntst.elf"
+    copy hello      "hello.elf"
 fi
 
 echo ""