Add pytest-like --pdb post-mortem debugging to r2x run

Summary

Add an opt-in --pdb option to r2x run that enters Python's post-mortem debugger when plugin execution fails, similar to pytest --pdb.

Proposed behavior

r2x run plugin <plugin-name> --pdb --input input.json
r2x run pipeline.yaml <pipeline-name> --pdb

• Enter pdb on uncaught Python exceptions, including AssertionError and KeyboardInterrupt.
• For pipelines, stop in the failing plugin rather than starting a debugger for every plugin.
• Preserve the original error and nonzero exit status after continue or quit.
• Do not trigger for exceptions caught by plugin code.
• Require an interactive terminal and fail clearly instead of hanging in CI or piped execution.
• Keep debugger prompts on stderr so JSON/plugin output on stdout remains usable.
• Ensure --no-stdout does not hide the debugger prompt.

Implementation notes

Use Python's standard-library pdb.post_mortem(traceback) through the existing PyO3 bridge; no new Python dependency is needed. PyO3 0.27 exposes PyErr::traceback(py), which should be called before the existing format_python_error conversion.

The bridge should also support plugin-returned Err values when their exception object has a __traceback__. The existing format_err_result path already inspects that attribute.

Avoid relying on pdb.pm()/sys.last_*; passing the traceback directly is compatible with the project's Python 3.11+ target and avoids mutating interpreter-global error state.

Acceptance criteria

• --pdb is accepted for direct plugin and pipeline run modes.
• A fixture plugin raising AssertionError opens an interactive PDB session at the failing frame.
• Continuing or quitting PDB returns the normal plugin failure with the expected exit status.
• Noninteractive invocation produces a clear diagnostic and does not block.
• Existing non---pdb behavior and stdout/JSON contracts remain unchanged.
• CLI help and documentation describe the option and the --input FILE recommendation for interactive debugging.

References: Python pdb.post_mortem, PyO3 PyErr, pytest --pdb.

Start at the r2x run option handling and existing PyO3 bridge, especially format_python_error and format_err_result; verify how direct plugin and pipeline failures are propagated. Add fixture coverage for AssertionError, interactive and noninteractive execution, stdout behavior, and exit status, then confirm CLI help documents --pdb and the --input FILE recommendation.