kernel: M1 shared-fate — Task.leader id, kill/signal re-keyed to the leader

Every task carries its process leader's id (main task: own id; threads:
copied from the spawner; kernel tasks: 0, never followed). process_kill and
process_signal resolve any member id to the leader and authorize against the
leader's supervisor, making both capabilities per-process. ProcessDescriptor
gains the leader field. No fan-out yet (docs/shared-fate-plan.md M1).

system/kernel/tests.zig

@@ -1430,7 +1430,7 @@ fn spawnFaultingProcess() ?u32 {
     architecture.mapUserPageInto(address_space, process.stack_base_virtual, stack_frame, true, false); // RW + NX
 
     // Supervised by the calling test task, so exitReasonOf can read the verdict.
-    const id = scheduler.spawnUserLocked(address_space, process.code_virtual, process.stack_base_virtual + abi.page_size, 0, 4, "fault-probe", scheduler.currentId(), null) orelse {
+    const id = scheduler.spawnUserLocked(address_space, process.code_virtual, process.stack_base_virtual + abi.page_size, 0, 4, "fault-probe", scheduler.currentId(), null, 0) orelse {
         architecture.destroyAddressSpace(address_space);
         return null;
     };