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.
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