Refactor DAG internals and lazy-load networkx

[tobiasraabe]

Summary

• replace the internal networkx DAG usage with a pytask-owned DAG implementation
• keep build_dag() and the visualization path returning a networkx.DiGraph via on-demand conversion
• make networkx optional and only import it on the DAG export boundary

Benchmarks

Compared this branch against origin/main in an isolated worktree after plain uv sync in both trees.

• pytask --help
  • current: 282.3 ms ± 43.0 ms
  • origin/main: 339.8 ms ± 27.3 ms
  • about 1.20x faster
• python -c "import pytask"
  • current: 284.5 ms ± 40.5 ms
  • origin/main: 320.1 ms ± 26.8 ms
  • about 1.13x faster

The current branch runtime environment had no installed networkx after uv sync, while origin/main still did.

Testing

• just typing
• uvx ruff check src/_pytask/dag.py src/_pytask/dag_command.py src/_pytask/dag_graph.py src/_pytask/dag_utils.py src/_pytask/execute.py src/_pytask/session.py src/_pytask/shared.py src/_pytask/mark/__init__.py tests/test_dag_command.py tests/test_dag_utils.py pyproject.toml docs/source/tutorials/visualizing_the_dag.md
• uvx ruff format --check src/_pytask/dag.py src/_pytask/dag_command.py src/_pytask/dag_graph.py src/_pytask/dag_utils.py src/_pytask/execute.py src/_pytask/session.py src/_pytask/shared.py src/_pytask/mark/__init__.py tests/test_dag_command.py tests/test_dag_utils.py
• uv run --group test pytest tests/test_dag_command.py tests/test_dag_utils.py tests/test_build.py tests/test_execute.py tests/test_mark.py tests/test_dag.py -q
• uv run --group test pytest tests/test_collect_command.py tests/test_execute.py tests/test_lockfile.py tests/test_profile.py -q