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.

system/services/device-manager/device-manager.zig

@@ -38,7 +38,7 @@ pub fn main() void {
     for (buffer[0..count]) |descriptor| {
         const driver_name = driverFor(descriptor.class) orelse continue;
         matched += 1;
-        if (runtime.system.spawn(driver_name)) {
+        if (runtime.system.spawn(driver_name) != null) {
             _ = runtime.system.write("device-manager: spawned ");
             _ = runtime.system.write(driver_name);
             _ = runtime.system.write("\n");