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/.gitignore

@@ -0,0 +1,4 @@
+target
+corpus
+artifacts
+Cargo.lock