nx_thread_spawn_test.nx

nx_thread_spawn_test.nx -- end-to-end test for __thread_clone: real OS thread spawn + atomic counter bumping + spin-join. Strategy: parent allocates a shared counter (mmap'd page so child sees the same memory), spawns a worker thread that bumps the counter N times via atomic FAA, then spin-waits until the counter hits N. No futex/condvar needed for this smoke -- just proves the child trampoline works and that atomics propagate cross-thread. Per cardinal end-to-end-or-dont-ship: this test EXERCISES the trampoline rather than just compiling it. Runs under qemu-riscv64-static which emulates clone() correctly.