kernel+tests: M4 shared-fate — nine group-death test cases, per-space arena cursors

Eight new QEMU cases (thread-fault-group, kill-threaded-group,
kill-via-worker-tid, racing-triggers, exit-group, leader-thread-exit,
thread-exit-solo, shm-mapping-ref) driving seven new thread-test modes; a
shared checkGroupDead asserts the contract everywhere: one notification,
badged with the leader, reason on the leader's record, no member listed,
claims released first.

The shm-mapping-ref case flushed out the per-task DMA/shared-memory arena
cursor bug directly (a sibling's regions mapped over the worker's), so both
cursors moved to the AddressSpaceRef like the mmap/MMIO cursors before them
(threading M7 pattern). Runtime gains Thread.tryExitCurrent for the leader
-EPERM refusal path. Docs updated: threading.md's shared-fate gap is closed,
process-management.md and process-lifecycle.md describe the leader re-key,
plan status = implemented. Full suite: 100/100.

docs/process-management.md

@@ -57,9 +57,13 @@ dangle even if the supervisor dies first.
 
 ### `process_kill(id) -> 0 / -ESRCH / -EPERM`
 
-Only the supervisor may kill; kernel tasks are not killable processes. Like a
-signal, delivery is prompt but asynchronous — 0 means the kill is accepted and
-irrevocable; the exit notification confirms completion.
+Only the supervisor may kill; kernel tasks are not killable processes. The kill
+is a **whole-process** kill ([shared-fate-plan.md](shared-fate-plan.md)): `id`
+may name any member of a threaded process — it resolves to the group's leader,
+authorization is checked against the *leader's* supervisor, and every thread
+dies. Like a signal, delivery is prompt but asynchronous — 0 means the kill is
+accepted and irrevocable; the exit notification (badged with the leader, posted
+once the last member is gone) confirms completion.
 
 ## How a kill lands (the kernel mechanics)