Record and expose how every process ends (M17.2)

The kernel records an ExitReason at all three death sites — clean exit,
fault (classified by vector), and process_kill — into a bounded ring
before the exit notification posts, so a supervisor's query never races
the notice. process_exit_reason is gated by the same supervisor check as
kill; runtime.process.exitReason is the stable interface. This is the
input restart policy reads (docs/process-lifecycle.md iron rule 2).

system/kernel/scheduler.zig

@@ -50,6 +50,10 @@ pub const Task = struct {
     // null. Holds its own reference, dropped when the notification is posted.
     // Opaque here for the same reason as `handles` below.
     exit_endpoint: ?*anyopaque = null,
+    // How this process ended — set by the death paths (exit, fault, kill) just
+    // before the reap records it for `process_exit_reason`. Meaningless while
+    // the task lives.
+    exit_reason: abi.ExitReason = .exited,
     // Set by process_kill on a task that is running on another core; the kernel
     // finishes the kill at that task's next system call or timer tick.
     kill_pending: bool = false,