feat(userland): add stlxstd C++ threading library with futex-based sync primitives

[FlareCoding]

Summary

Builds the userland synchronization stack on top of the kernel futex primitive (merged in #123). Three layers:

C Layer (libstlx extensions)

• stlx_futex_wait/wake/wake_all — thin syscall wrappers for SYS_FUTEX_WAIT/WAKE/WAKE_ALL
• stlx_mutex_t — three-state mutex (Drepper algorithm): atomic CAS fast path, futex slow path
• stlx_cond_t — sequence-counter condition variable
• stlx_barrier_t — generation-counter reusable barrier

C++ Layer (new libstlxcxx)

• stlxstd::thread — RAII thread with mmap'd stack, join/detach, type-erased callable support
• stlxstd::mutex — wraps stlx_mutex_t
• stlxstd::lock_guard<M> / unique_lock<M> — RAII lock management
• stlxstd::condition_variable — wraps stlx_cond_t with predicated wait
• stlxstd::barrier — wraps stlx_barrier_t

Test App

• synctest — validates all primitives: mutex stress (8 threads × 10K iterations), condvar producer/consumer, barrier synchronization, thread join/detach, multi-mutex independence

Kernel Fix

• TLS inheritance for user threads — create_user_thread now inherits the creator's TLS base instead of setting it to 0. Without this, any %fs-relative access (musl errno, malloc, etc.) in a child thread would null-deref.

Design Decisions

• Uncontended mutex lock/unlock is a single atomic CAS — zero syscalls
• stlxstd::thread detach intentionally leaks the stack (freed at process exit) — can't munmap a stack you're running on
• Thread callable dispatch uses function pointer (no virtual/RTTI dependency)
• Apps opt into libstlxcxx via APP_LIBS := stlxcxx in their Makefile
• All libstlx C headers now have extern "C" guards for C++ linkage

Testing

• All 509 kernel unit tests pass
• Both x86_64 and aarch64 userland builds succeed (40/40 apps)
• synctest passes all 6 tests on Stellux x86_64

  

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