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_dag_execution.rs

@@ -0,0 +1,120 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use dagx::{task, DagRunner};
+
+fuzz_target!(|data: &[u8]| {
+    // Only fuzz with reasonable data sizes to avoid timeouts
+    if data.is_empty() || data.len() > 1000 {
+        return;
+    }
+
+    // Convert bytes to values for tasks
+    let value1 = data.get(0).copied().unwrap_or(0) as i32;
+    let value2 = data.get(1).copied().unwrap_or(0) as i32;
+    let value3 = data.get(2).copied().unwrap_or(0) as i32;
+
+    // Test simple linear DAG execution
+    {
+        struct Source1 { val: i32 }
+        #[task]
+        impl Source1 {
+            async fn run(&self) -> i32 { self.val }
+        }
+
+        struct Source2 { val: i32 }
+        #[task]
+        impl Source2 {
+            async fn run(&self) -> i32 { self.val }
+        }
+
+        struct Compute;
+        #[task]
+        impl Compute {
+            async fn run(a: &i32, b: &i32) -> i32 {
+                a.wrapping_add(*b)
+            }
+        }
+
+        let dag = DagRunner::new();
+        let s1 = dag.add_task(Source1 { val: value1 });
+        let s2 = dag.add_task(Source2 { val: value2 });
+        let compute = dag.add_task(Compute).depends_on((&s1, &s2));
+
+        let rt = tokio::runtime::Runtime::new().unwrap();
+        if let Ok(()) = rt.block_on(async {
+            dag.run(|fut| { tokio::spawn(fut); }).await
+        }) {
+            // Verify output
+            if let Ok(result) = dag.get(compute) {
+                assert_eq!(result, value1.wrapping_add(value2));
+            }
+        }
+    }
+
+    // Test fan-out pattern
+    {
+        struct Source { val: i32 }
+        #[task]
+        impl Source {
+            async fn run(&self) -> i32 { self.val }
+        }
+
+        struct Double;
+        #[task]
+        impl Double {
+            async fn run(input: &i32) -> i32 {
+                input.wrapping_mul(2)
+            }
+        }
+
+        struct Triple;
+        #[task]
+        impl Triple {
+            async fn run(input: &i32) -> i32 {
+                input.wrapping_mul(3)
+            }
+        }
+
+        let dag = DagRunner::new();
+        let source = dag.add_task(Source { val: value3 });
+        let double = dag.add_task(Double).depends_on(&source);
+        let triple = dag.add_task(Triple).depends_on(&source);
+
+        let rt = tokio::runtime::Runtime::new().unwrap();
+        if let Ok(()) = rt.block_on(async {
+            dag.run(|fut| { tokio::spawn(fut); }).await
+        }) {
+            // Verify outputs
+            if let (Ok(d), Ok(t)) = (dag.get(double), dag.get(triple)) {
+                assert_eq!(d, value3.wrapping_mul(2));
+                assert_eq!(t, value3.wrapping_mul(3));
+            }
+        }
+    }
+
+    // Test error-free execution with various data
+    if data.len() >= 4 {
+        struct DataTask { data: Vec<u8> }
+        #[task]
+        impl DataTask {
+            async fn run(&self) -> Vec<u8> {
+                self.data.clone()
+            }
+        }
+
+        let dag = DagRunner::new();
+        let task = dag.add_task(DataTask {
+            data: data[..4.min(data.len())].to_vec()
+        });
+
+        let rt = tokio::runtime::Runtime::new().unwrap();
+        if let Ok(()) = rt.block_on(async {
+            dag.run(|fut| { tokio::spawn(fut); }).await
+        }) {
+            if let Ok(result) = dag.get(task) {
+                assert_eq!(result.len(), 4.min(data.len()));
+            }
+        }
+    }
+});