refactor: move task_fn into its own crate

tests/parallelism/concurrent.rs

@@ -1,7 +1,7 @@
 //! Concurrent access tests
 
-use dagx::task_fn;
 use dagx::{task, DagRunner};
+use dagx_test::task_fn;
 
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;

tests/parallelism/spawning.rs

@@ -1,7 +1,7 @@
 //! Tests for spawner functionality
 
-use dagx::task_fn;
 use dagx::{DagResult, DagRunner};
+use dagx_test::task_fn;
 
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;

tests/parallelism/threads.rs

@@ -1,7 +1,7 @@
 //! Tests for multi-threading and concurrent execution
 
-use dagx::task_fn;
 use dagx::{task, DagResult, DagRunner, TaskHandle};
+use dagx_test::task_fn;
 
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::Arc;

tests/parallelism/timing.rs

@@ -1,6 +1,7 @@
 //! Tests for timing and speedup verification
 
-use dagx::{task, task_fn, DagResult, DagRunner};
+use dagx::{task, DagResult, DagRunner};
+use dagx_test::task_fn;
 
 use std::{
     sync::{Arc, Mutex},