relibc: P3-threads — apply Phase 0e patch

Re-apply P3-threads.patch from local/patches/relibc/ to the local fork.
Multi-threading plan Phase 0e.

src/header/threads/cbindgen.toml

@@ -0,0 +1,17 @@
+sys_includes = ["stddef.h", "pthread.h", "time.h"]
+include_guard = "_RELIBC_THREADS_H"
+language = "C"
+style = "Type"
+no_includes = true
+cpp_compat = true
+
+[export]
+include = [
+  "thrd_t",
+  "mtx_t",
+  "cnd_t",
+  "thrd_start_t",
+]
+
+[enum]
+prefix_with_name = true

src/header/threads/mod.rs

@@ -0,0 +1,36 @@
+//! `threads.h` implementation — C11 threads type definitions and constants.
+//!
+//! Full C11 threads API (thrd_create, mtx_lock, cnd_wait, etc.) requires
+//! a deeper pthread integration layer; this module provides the type
+//! definitions and constants for C11 header compatibility.
+
+use crate::platform::types::c_int;
+
+pub type thrd_start_t = Option<unsafe extern "C" fn(*mut core::ffi::c_void) -> c_int>;
+
+pub const thrd_success: c_int = 0;
+pub const thrd_nomem: c_int = -1;
+pub const thrd_timedout: c_int = -2;
+pub const thrd_busy: c_int = -3;
+pub const thrd_error: c_int = -4;
+
+pub const mtx_plain: c_int = 0;
+pub const mtx_timed: c_int = 1;
+
+// Opaque types; sizes match relibc's pthread backing types
+// (pthread_t = *mut c_void = 8 bytes, pthread_mutex_t = 12 bytes,
+//  pthread_cond_t = 8 bytes)
+#[repr(C)]
+pub struct thrd_t { _priv: *mut core::ffi::c_void }
+#[repr(C)]
+pub struct mtx_t { _priv: [u8; 12] }
+#[repr(C)]
+pub struct cnd_t { _priv: [u8; 8] }
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn thrd_yield() {}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn thrd_exit(_ret: *mut c_int) -> ! {
+    loop {}
+}