test: add comprehensive testing infrastructure (fuzzing, proptest, mutation)

Implements dagx-11: Add comprehensive testing infrastructure including
fuzzing, property-based testing, and mutation testing configuration.

**Fuzzing Infrastructure:**
- Created fuzz/ directory with cargo-fuzz setup
- 4 fuzz targets covering different scenarios:
  - fuzz_dag_execution: Basic DAG execution with various patterns
  - fuzz_dependencies: Dependency management (linear, diamond, complex)
  - fuzz_type_safety: Type-safe output retrieval
  - fuzz_concurrent: Concurrent execution and thread safety
- Comprehensive README with usage instructions
- Integrated with justfile (fuzz, fuzz-all, fuzz-extended, fuzz-build)

**Property-Based Testing:**
- Added proptest dependency to Cargo.toml
- Created tests/property_tests.rs with 14 property tests:
  - Value preservation through DAG
  - Linear chain correctness
  - Fan-out pattern verification
  - Diamond pattern correctness
  - String, Vec, Option, Result handling
  - HashMap and custom struct preservation
  - DAG isolation
  - Concurrent execution scaling
- Integrated with justfile (proptest recipe)

**Mutation Testing:**
- Created .cargo-mutants.toml configuration
- Focused on critical functions:
  - DagRunner::run and ::get
  - TaskBuilder::depends_on
  - ExecutableNode::execute_with_listeners
  - ExtractFromListeners::extract
  - DepsTuple::to_node_ids
- Excluded low-value mutations (fmt, clone, default)
- Configured for nextest and parallel execution
- Already integrated via existing justfile mutants recipe

**Justfile Updates:**
- Added fuzzing section with 5 new recipes
- Added proptest recipe for property-based tests
- All recipes follow existing patterns and naming conventions

Similar setup to ../announcement project but tailored for DAG-specific testing.

fuzz/fuzz_targets/fuzz_concurrent.rs

@@ -0,0 +1,137 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use dagx::{task, DagRunner};
+use std::sync::Arc;
+
+fuzz_target!(|data: &[u8]| {
+    if data.is_empty() || data.len() > 50 {
+        return;
+    }
+
+    let rt = tokio::runtime::Runtime::new().unwrap();
+
+    // Test concurrent task execution
+    struct SlowTask { val: i32, delay_ms: u64 }
+    #[task]
+    impl SlowTask {
+        async fn run(&self) -> i32 {
+            tokio::time::sleep(tokio::time::Duration::from_millis(self.delay_ms)).await;
+            self.val
+        }
+    }
+
+    struct FastTask { val: i32 }
+    #[task]
+    impl FastTask {
+        async fn run(&self) -> i32 { self.val }
+    }
+
+    // Test concurrent independent tasks
+    {
+        let dag = DagRunner::new();
+
+        // Create multiple independent tasks
+        let num_tasks = (data[0] % 5 + 1) as usize;
+        let mut tasks = Vec::new();
+
+        for i in 0..num_tasks {
+            let val = data.get(i).copied().unwrap_or(0) as i32;
+            let delay = (val.abs() as u64 % 10) + 1;
+
+            let task = dag.add_task(SlowTask { val, delay_ms: delay });
+            tasks.push((task, val));
+        }
+
+        if rt.block_on(async {
+            dag.run(|fut| { tokio::spawn(fut); }).await
+        }).is_ok() {
+            // Verify all tasks completed with correct values
+            for (handle, expected) in tasks {
+                if let Ok(result) = dag.get(handle) {
+                    assert_eq!(result, expected);
+                }
+            }
+        }
+    }
+
+    // Test concurrent dependent tasks
+    {
+        let dag = DagRunner::new();
+
+        let source = dag.add_task(FastTask { val: data[0] as i32 });
+
+        // Create multiple tasks that depend on source
+        struct AddOne;
+        #[task]
+        impl AddOne {
+            async fn run(input: &i32) -> i32 {
+                input.wrapping_add(1)
+            }
+        }
+
+        struct AddTwo;
+        #[task]
+        impl AddTwo {
+            async fn run(input: &i32) -> i32 {
+                input.wrapping_add(2)
+            }
+        }
+
+        struct AddThree;
+        #[task]
+        impl AddThree {
+            async fn run(input: &i32) -> i32 {
+                input.wrapping_add(3)
+            }
+        }
+
+        let t1 = dag.add_task(AddOne).depends_on(&source);
+        let t2 = dag.add_task(AddTwo).depends_on(&source);
+        let t3 = dag.add_task(AddThree).depends_on(&source);
+
+        if rt.block_on(async {
+            dag.run(|fut| { tokio::spawn(fut); }).await
+        }).is_ok() {
+            let base = data[0] as i32;
+            if let Ok(r1) = dag.get(t1) {
+                assert_eq!(r1, base.wrapping_add(1));
+            }
+            if let Ok(r2) = dag.get(t2) {
+                assert_eq!(r2, base.wrapping_add(2));
+            }
+            if let Ok(r3) = dag.get(t3) {
+                assert_eq!(r3, base.wrapping_add(3));
+            }
+        }
+    }
+
+    // Test Arc sharing across concurrent tasks
+    {
+        let dag = Arc::new(DagRunner::new());
+
+        struct SharedTask { val: i32 }
+        #[task]
+        impl SharedTask {
+            async fn run(&self) -> i32 { self.val }
+        }
+
+        let task = dag.add_task(SharedTask { val: data[0] as i32 });
+        let task_copy = task;
+
+        let dag1 = Arc::clone(&dag);
+        let dag2 = Arc::clone(&dag);
+
+        if rt.block_on(async {
+            dag.run(|fut| { tokio::spawn(fut); }).await
+        }).is_ok() {
+            // Multiple threads can access result
+            if let Ok(r1) = dag1.get(task_copy) {
+                if let Ok(r2) = dag2.get(task_copy) {
+                    assert_eq!(r1, r2);
+                    assert_eq!(r1, data[0] as i32);
+                }
+            }
+        }
+    }
+});