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.

build.zig

@@ -300,6 +300,7 @@ pub fn build(b: *std.Build) void {
     const bus_exe = addUserBinary(b, kernel_target, runtime_module, posix_module, mmio_module, "bus", "system/drivers/bus/bus.zig");
     const device_manager_exe = addUserBinary(b, kernel_target, runtime_module, posix_module, mmio_module, "device-manager", "system/services/device-manager/device-manager.zig");
     const args_echo_exe = addUserBinary(b, kernel_target, runtime_module, posix_module, mmio_module, "args-echo", "system/services/args-echo/args-echo.zig");
+    const process_test_exe = addUserBinary(b, kernel_target, runtime_module, posix_module, mmio_module, "process-test", "system/services/process-test/process-test.zig");
 
     // Pack the user binaries into the initial_ramdisk image with the host-side Python tool
     // (the container format is trivial, and Python sidesteps std API churn). Args:
@@ -319,6 +320,8 @@ pub fn build(b: *std.Build) void {
     mk_run.addFileArg(device_manager_exe.getEmittedBin());
     mk_run.addArg("args-echo");
     mk_run.addFileArg(args_echo_exe.getEmittedBin());
+    mk_run.addArg("process-test");
+    mk_run.addFileArg(process_test_exe.getEmittedBin());
 
     // Also install the packed binaries to their FHS homes, so zig-out is a true image
     // of the filesystem — even though at boot they arrive inside the initial-ramdisk.