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

crates/domain/src/timer.rs

@@ -18,6 +18,9 @@ pub struct Target {
     pub badge: Badge,
 }
 
+/// How many may watch one process die: its parent and init, with room.
+pub const MAX_WATCHERS: usize = 4;
+
 pub struct Timers {
     deadlines: [Option<(Ticks, Target)>; MAX_PROCESSES],
 }