feat: dagrunner, initial version

tests/README.md

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+# Test Suite
+
+## Purpose
+
+Integration and performance tests for dagx. Unit tests live in `src/{module}/tests.rs`.
+
+## Directory Structure
+
+```
+tests/
+├── common/          # Shared test utilities
+├── dependencies/    # Dependency resolution tests
+├── execution/       # Task execution and DAG running
+├── parallelism/     # Parallel execution verification
+├── panic/           # Panic handling and isolation
+├── performance/     # Stress tests and benchmarks
+├── runtimes/        # Runtime compatibility tests
+├── task_patterns/   # Common task pattern tests
+├── type_safety/     # Type system validation
+└── lib_tests.rs     # Main test entry point
+```
+
+## Running Tests
+
+```bash
+# All tests (unit + integration)
+cargo test
+
+# Unit tests only
+cargo test --lib
+
+# Integration tests only
+cargo test --test lib_tests
+
+# Specific category
+cargo test --test lib_tests -- parallelism::
+cargo test --test lib_tests -- execution::
+
+# With output
+cargo test -- --nocapture
+
+# Single threaded (for debugging)
+cargo test -- --test-threads=1
+
+# Performance tests (excluded by default)
+cargo test --release -- --ignored
+```
+
+## Test Categories
+
+### Unit Tests (`src/{module}/tests.rs`)
+
+- Module-specific logic
+- Edge cases and error conditions
+- Type implementations
+
+### Integration Tests (`tests/`)
+
+- **execution**: End-to-end DAG execution
+- **dependencies**: Complex dependency graphs
+- **parallelism**: Concurrent task execution
+- **panic**: Error propagation and isolation
+- **performance**: Large DAGs and stress tests
+- **runtimes**: Tokio, async-std, smol compatibility
+- **task_patterns**: Common usage patterns
+- **type_safety**: Type system guarantees
+
+## Writing Tests
+
+Integration tests use common utilities from `tests/common/mod.rs`.
+
+Unit tests should:
+
+- Test single units of functionality
+- Mock external dependencies
+- Cover error cases
+
+Integration tests should:
+
+- Test component interactions
+- Use real implementations
+- Verify end-to-end behavior