refactor: move task_fn into its own crate

tests/boundaries/deep_chains.rs

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

tests/boundaries/type_limits.rs

@@ -1,7 +1,7 @@
 //! Tests for type system edge cases and limits
 
-use dagx::task_fn;
 use dagx::{DagResult, DagRunner, TaskHandle};
+use dagx_test::task_fn;
 
 use std::marker::PhantomData;
 

tests/boundaries/zero_tasks.rs

@@ -1,7 +1,7 @@
 //! Tests for empty DAGs and minimal task configurations
 
-use dagx::task_fn;
 use dagx::{task, DagResult, DagRunner};
+use dagx_test::task_fn;
 
 use tokio::task::yield_now;