Add process management: enumerate, supervisor-gated kill, exit notifications

process_enumerate snapshots the task table (the device_enumerate shape, so
ps is a user program); system_spawn returns the child id, records the caller
as supervisor, and takes an exit endpoint; process_kill is allowed only for
the supervisor. Every death — exit, fault, or kill — posts a child-exit badge
to that endpoint (the IRQ-as-IPC pattern as SIGCHLD). A target caught off-CPU
is reaped in place; a running one is condemned and finished at its next
system call or tick, guarded so teardown never lands mid-kernel-operation.
Tested by process-list, process-kill, and supervision (a ring-3 supervisor
exercising the whole surface); design notes in docs/process-management.md.

library/runtime/ipc.zig

@@ -84,6 +84,11 @@ pub fn call(h: Handle, message: []const u8, reply: []u8) CallError!usize {
 /// GSI. See `isNotification`.
 pub const notify_badge_bit: u64 = abi.notify_badge_bit;
 
+/// Set alongside `notify_badge_bit` when the notification is a **child-exit
+/// notice** — a process this one spawned (with an exit endpoint) has ended —
+/// rather than a device interrupt. The low bits carry the child's process id.
+pub const notify_exit_bit: u64 = abi.notify_exit_bit;
+
 /// The result of a `replyWait`: the request length, the sender's badge (a task id, or
 /// an IRQ notification if the high bit is set), and any capability the request carried.
 pub const Received = struct {
@@ -91,16 +96,29 @@ pub const Received = struct {
     badge: u64,
     cap: ?Handle,
 
-    /// True if this wake-up was a device interrupt, not a client request. A driver's
-    /// event loop branches on this; there is no reply owed on the notification path.
+    /// True if this wake-up was an asynchronous notification (a device interrupt
+    /// or a child-exit notice), not a client request. An event loop branches on
+    /// this; there is no reply owed on the notification path.
     pub fn isNotification(self: Received) bool {
         return self.badge & notify_badge_bit != 0;
     }
 
-    /// The interrupt source (a GSI), meaningful only when `isNotification`.
+    /// True if this wake-up tells of a supervised child's end — the notification
+    /// requested by passing an exit endpoint to `system.spawnSupervised`.
+    pub fn isChildExit(self: Received) bool {
+        return self.isNotification() and self.badge & notify_exit_bit != 0;
+    }
+
+    /// The interrupt source (a GSI), meaningful only when `isNotification` and
+    /// not `isChildExit`.
     pub fn source(self: Received) u64 {
         return self.badge & ~notify_badge_bit;
     }
+
+    /// The ended child's process id, meaningful only when `isChildExit`.
+    pub fn childProcessId(self: Received) u32 {
+        return @intCast(self.badge & ~(notify_badge_bit | notify_exit_bit));
+    }
 };
 
 /// Server side of IPC_ReplyWait: deliver `reply` to the client last received (if any,