Add POSIX PSE51/PSE52 compatibility layer

[jserv]

This implementation provides support for the IEEE Std 1003.13-2003 POSIX real-time profiles on the F9 microkernel as an additional API layer for application development.

PSE51 (Minimal Realtime System Profile):

• Threads: pthread_create/join/detach/exit/self/equal, cancellation
• Mutexes: normal/recursive/errorcheck with timedlock support
• Condition variables: wait/timedwait/signal/broadcast
• Semaphores: init/wait/trywait/post/getvalue/destroy
• Time: clock_gettime/nanosleep using L4_SystemClock
• Signals: per-thread state with sigmask/sigaction/sigwait

PSE52 (Realtime Controller) partial:

• Spinlocks: init/lock/trylock/unlock/destroy
• Read-write locks and barriers (stubs)

Implementation highlights:

• Notification-based blocking via L4_NotifyWait/Post/Clear
• UTCB-keyed cancel_table for thread cancellation state
• Thread lifecycle tracking (CREATED/RUNNING/EXITED)
• Proper timeout handling with TIMEOUT_BIT clearing
• Atomic sem_post overflow check to prevent TOCTOU race

Kernel changes:

• Add SYS_NOTIFY_CLEAR syscall for IRQ-safe bit clearing
• Fix race condition in sys_notify_post notification delivery

Close #98

---

Summary by cubic

Adds a POSIX API layer (PSE51 with partial PSE52) and kernel notification syscalls, enabling pthreads, semaphores, signals, and time on F9. Includes a POSIX test app with CI integration and fixes a notification delivery race.

• New Features

  • POSIX PSE51: pthreads (create/join/detach/cancel), mutexes (normal/recursive/errorcheck + timedlock), condition variables, semaphores, signals, time (clock_gettime, nanosleep).
  • PSE52 partial: read-write locks and barriers.
  • New notification syscalls: SYS_NOTIFY_WAIT/POST/CLEAR with T_NOTIFY_BLOCKED state and proper wake semantics.
  • L4_SystemClock backed by atomic ktimer_get_now; improved IPC MR copying.
  • POSIX compliance app and unified test framework; CI runs “Kernel Test Suite” and “POSIX Compliance” in QEMU.

• Migration

  • Replace legacy libposix headers with posix/* (pthread.h, semaphore.h, signal.h, time.h, sys/types.h).
  • Use L4_NotifyWait/L4_NotifyPost/L4_NotifyClear for notification-based blocking.
  • Enable CONFIG_USER_APP_POSIX to run the new test suite; user apps build now includes posix.

^{Written for commit 8283605. Summary will update on new commits.}