fix(process): serialize non-leader exec handoff (#2156)

Model non-leader exec as an owned group-exec transaction that serializes sibling teardown, old-leader exit, fatal cancellation, identity migration, and observer visibility.

Atomically preserve PID/TGID/PGID/SID membership, raw-PID lookup, cgroup placement, parent-child ownership, ptrace relations, pidfd identity, session state, and accumulated resource usage across the leader handoff.

Align wait, SIGCHLD, ptrace exit notification, fatal thread-group signals, and process-group delivery with Linux 6.6 semantics. Preserve exited-thread CPU accounting and publish child-exit state before waking signal and wait observers.

Serialize controlling-TTY ownership and PTY peer lifetime transitions to prevent stale session updates, duplicate group signals, premature ctty removal, and slave-open races.

Add deterministic dunitest coverage for non-leader exec identity, pidfd visibility, fatal-signal races, wait ownership, resource accounting, process groups, ptrace siginfo, and PTY lifetime behavior.

Fixes #2153

Signed-off-by: longjin longjin@dragonos.org

轻量级云原生内核

DragonOS

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