Add checkpointing support for long-running mechanism synthesis in dpsynth:

[copybara-service]

Add checkpointing support for long-running mechanism synthesis in dpsynth:

• Introduce Checkpointer (dpsynth/checkpoint.py) leveraging etils.epath and mbi pytree serialization to persist and restore intermediate mechanism artifacts (.npz files) when a working_dir is specified, and acting as a transparent no-op when disabled (working_dir=None).
• Add working_dir property to MechanismConfig and SWIFTConfig.
• Implement stage-level resume in SWIFT:
  • Fast-path resume to synthetic data generation when both model.npz and measurements.npz exist.
  • Intermediate resume reusing cached exact marginals (marginals.npz) to skip from_projectable.
  • Checkpointing measurements.npz after query measurement and model.npz after PGM estimation.
• Add unit tests for Checkpointer and stage resumes in SWIFT.

[MuhammadNiazAli]

The import reorganization and new checkpoint_lib import look clean. The new working_dir config field is well-documented in the docstring, including the None-disables-checkpointing behavior, which is good.

I've only reviewed swift.py here there are 3 more changed files I haven't looked at yet, so holding off on approving until I go through those too.

[MuhammadNiazAli]

The two-stage resume logic in swift.py reads well:

Full resume (model.npz + measurements.npz both exist) skips straight to synthesis using the cached model/measurements, correctly falling back to initial_measurements over the checkpointed ones when both are available (total_src = initial_measurements if initial_measurements else measurements).
Partial resume (marginals only) — loads marginals.npz if present, otherwise computes via mbi.CliqueVector.from_projectable and saves it before continuing. The assert answers is not None after load is a nice defensive touch even though exists() already gated it.

One thing I couldn't verify from the diff alone: checkpointer = checkpoint_lib.Checkpointer(self.config.working_dir) is constructed unconditionally, and the docstring says working_dir=None disables checkpointing. Worth confirming Checkpointer(None) actually makes exists() always return False and save()/load() no-ops otherwise this line would break for anyone not opting into checkpointing.

Also noticed in the full-resume branch, phase_times stays {} since none of the common.timed(...) blocks run that's expected, but if any downstream diagnostics code assumes specific phase keys are always present, it'd be worth a quick check.

test_checkpointing_saves_and_resumes in swift_test.py is a good addition — real temp dir, real save/load round-trip rather than mocking. The unused-looking from etils import epath import is that used later in the test for path handling, or is it dead? Couldn't tell from what's shown.