feat(mt): move serial advance orchestration into Rust

Move the serial turn increment, broadcast, and unlock ordering into a Rust
entry point. C retains the pthread condition variable and mutex operations
behind callbacks, while Rust preserves wrapping unsigned job-counter
semantics and the required broadcast-before-unlock contract. Add ABI layout
checks and focused tests for ordinary and wrapping advancement.

Test Plan:
- cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml zstdmt_compress::tests::serial_ -- --nocapture
- ulimit -v 41943040 && make -j1

rust/src/zstdmt_compress.rs

@@ -91,6 +91,8 @@ pub type ZSTDMT_serialUpdateChecksumFn = unsafe extern "C" fn(*mut c_void, *cons
 pub type ZSTDMT_serialAdvanceFn = unsafe extern "C" fn(*mut c_void);
 pub type ZSTDMT_serialStateLockFn = unsafe extern "C" fn(*mut c_void);
 pub type ZSTDMT_serialStateWaitFn = unsafe extern "C" fn(*mut c_void);
+pub type ZSTDMT_serialStateBroadcastFn = unsafe extern "C" fn(*mut c_void);
+pub type ZSTDMT_serialStateUnlockFn = unsafe extern "C" fn(*mut c_void);
 
 /// Projection for the MT serial turn wait.  Rust owns the lock/wait loop and
 /// comparison; C retains the pthread mutex and condition variable behind the
@@ -143,6 +145,49 @@ pub unsafe extern "C" fn ZSTDMT_rust_serialStateWaitForTurn(
     }
 }
 
+/// Projection for advancing the MT serial turn.  Rust owns the increment and
+/// callback order; C retains the condition variable broadcast and mutex
+/// unlock operations behind the callbacks.
+#[repr(C)]
+pub struct ZSTDMT_RustSerialAdvanceState {
+    callback_context: *mut c_void,
+    next_job_id: *mut c_uint,
+    broadcast: Option<ZSTDMT_serialStateBroadcastFn>,
+    unlock: Option<ZSTDMT_serialStateUnlockFn>,
+}
+
+const _: () = {
+    assert!(size_of::<ZSTDMT_serialStateBroadcastFn>() == size_of::<usize>());
+    assert!(size_of::<ZSTDMT_serialStateUnlockFn>() == size_of::<usize>());
+    assert!(offset_of!(ZSTDMT_RustSerialAdvanceState, callback_context) == 0);
+    assert!(offset_of!(ZSTDMT_RustSerialAdvanceState, next_job_id) == size_of::<usize>());
+    assert!(offset_of!(ZSTDMT_RustSerialAdvanceState, broadcast) == 2 * size_of::<usize>());
+    assert!(offset_of!(ZSTDMT_RustSerialAdvanceState, unlock) == 3 * size_of::<usize>());
+    assert!(size_of::<ZSTDMT_RustSerialAdvanceState>() == size_of::<[usize; 4]>());
+};
+
+#[no_mangle]
+pub unsafe extern "C" fn ZSTDMT_rust_serialStateAdvance(
+    state: *const ZSTDMT_RustSerialAdvanceState,
+) {
+    if state.is_null() {
+        return;
+    }
+    let state = unsafe { &*state };
+    if state.callback_context.is_null() || state.next_job_id.is_null() {
+        return;
+    }
+    let (Some(broadcast), Some(unlock)) = (state.broadcast, state.unlock) else {
+        return;
+    };
+
+    unsafe {
+        *state.next_job_id = (*state.next_job_id).wrapping_add(1);
+        broadcast(state.callback_context);
+        unlock(state.callback_context);
+    }
+}
+
 #[repr(C)]
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
 pub struct ZSTDMT_serialStateEnsureFinishedResult {
@@ -3540,6 +3585,38 @@ mod tests {
         }
     }
 
+    struct SerialAdvanceTestContext {
+        events: Vec<&'static str>,
+    }
+
+    unsafe extern "C" fn serial_advance_test_broadcast(context: *mut c_void) {
+        unsafe {
+            (*context.cast::<SerialAdvanceTestContext>())
+                .events
+                .push("broadcast")
+        };
+    }
+
+    unsafe extern "C" fn serial_advance_test_unlock(context: *mut c_void) {
+        unsafe {
+            (*context.cast::<SerialAdvanceTestContext>())
+                .events
+                .push("unlock")
+        };
+    }
+
+    fn serial_advance_test_state(
+        context: &mut SerialAdvanceTestContext,
+        next_job_id: &mut c_uint,
+    ) -> ZSTDMT_RustSerialAdvanceState {
+        ZSTDMT_RustSerialAdvanceState {
+            callback_context: context as *mut _ as *mut c_void,
+            next_job_id,
+            broadcast: Some(serial_advance_test_broadcast),
+            unlock: Some(serial_advance_test_unlock),
+        }
+    }
+
     unsafe extern "C" fn wait_for_ldm_test_lock(context: *mut c_void) {
         unsafe {
             (*context.cast::<WaitForLdmTestContext>())
@@ -3661,6 +3738,30 @@ mod tests {
         assert_eq!(context.next_job_id, 5);
     }
 
+    #[test]
+    fn serial_advance_increments_before_broadcast_and_unlock() {
+        let mut context = SerialAdvanceTestContext { events: Vec::new() };
+        let mut next_job_id = 7;
+        let state = serial_advance_test_state(&mut context, &mut next_job_id);
+
+        unsafe { ZSTDMT_rust_serialStateAdvance(&state) };
+
+        assert_eq!(next_job_id, 8);
+        assert_eq!(context.events, vec!["broadcast", "unlock"]);
+    }
+
+    #[test]
+    fn serial_advance_wraps_the_job_counter_like_c_unsigned_arithmetic() {
+        let mut context = SerialAdvanceTestContext { events: Vec::new() };
+        let mut next_job_id = c_uint::MAX;
+        let state = serial_advance_test_state(&mut context, &mut next_job_id);
+
+        unsafe { ZSTDMT_rust_serialStateAdvance(&state) };
+
+        assert_eq!(next_job_id, 0);
+        assert_eq!(context.events, vec!["broadcast", "unlock"]);
+    }
+
     fn record_compression_job_event(state: &Rc<RefCell<MockCompressionJob>>, event: &'static str) {
         state.borrow_mut().events.push(event);
     }