Implements Linux-style execve de_thread semantics and hands non-leader execs to the leader

[jserv]

Summary by cubic

Implements Linux-style execve teardown with leader handoff so new images start single‑threaded and we never reset guest memory under live threads. Also fixes a clone region lookup race and adds per‑test hang stack sampling for timeouts.

• Implements de_thread and hands non‑leader execve to the leader; replaces ad‑hoc checks with thread_stop_requested and adds thread_wake_all_blocked; guest teardown and exec use it to wake futex, poll, vCPU, and internal waiters.
• Futex and timers: waits now watch thread_stop_requested and advance only ITIMER_REAL during host waits to avoid charging virtual timers; bounded backoff loops materialize SIGALRM promptly.
• Poll/inotify/FUSE/time: finite waits run in slices and always carry the wakeup pipe; blocking reads and sleeps exit on thread_stop_requested instead of waiting out guest deadlines.
• I/O and locks: introduces io_retry_backoff and bounded retries; converts blocking semop, flock, F_SETLKW, and O_WRONLY FIFO open into poll‑with‑retry so teardown can interrupt without parking in un‑wakeable host calls.
• Fork interaction: serializes de_thread with the fork barrier/snapshot; thread_quiesce_siblings refuses new quiet windows once teardown is pending, callers abort with EINTR; teardown joins are bounded and avoid double‑teardown.
• Concurrency fix: clone’s resolve_clone_stack_range now reads guest regions under mmap_lock to stop races with concurrent mmap/munmap.
• Tests/harness: adds test-threaded-exec; integrates a watchdog stack sampler to capture hangs near timeouts; Makefile builds the new test.

^{Written for commit 5ca06a8. Summary will update on new commits.}