Add checkpoints to PopulationTrips so runs restart from the last computed iteration

[FlxPo]

This PR adds crash-safe checkpointing and automatic resume to PopulationTrips, improves determinism when resuming by rehydrating the congestion snapshot pointer, and introduces optional debug logging for the congestion pipeline. It also fixes a regex escape warning.

Motivation

• Long PopulationTrips runs can crash mid-iteration; restarting from scratch is expensive.
• Resume must be deterministic: after restart, the in-memory “current congestion snapshot” pointer must be restored or results can diverge.
• Investigating congestion-related instabilities needs better observability.

Changes

• Checkpointing + auto-resume

  • PopulationTrips now saves a checkpoint after each completed iteration (current states, remaining sinks, RNG state).
  • On start, if checkpoints exist for the same run key, the model resumes from the latest completed iteration.
  • Temp folders are preserved on resume; partial artifacts beyond the checkpoint are pruned.

• Checkpoint lifecycle integrated with remove

  • PopulationTrips.remove(remove_checkpoints=True) deletes output caches and the checkpoints for that specific run key (weekday + weekend).

• Clean resume helpers

  • Added mobility/choice_models/population_trips_resume.py to keep population_trips.py readable:
    • compute_resume_plan
    • try_load_checkpoint
    • restore_state_or_fresh_start
    • prune_tmp_artifacts
    • rehydrate_congestion_snapshot
  • Added Google-style docstrings for these helpers.
  • Added log lines to make the resume decision visible (“No checkpoint found…” / “Latest checkpoint found…”).

• Deterministic congestion resume (clean API)

  • Added PathTravelCosts.apply_flow_snapshot(flow_asset) and refactored snapshot application behind _apply_flow_snapshot(...).
  • Resume now calls apply_flow_snapshot(...) instead of doing a dummy update(...) call with empty flows.

• Optional congestion debug logs

  • When MOBILITY_DEBUG_CONGESTION=1, logs include:
    • flow asset cache hits/writes with hashes and row counts
    • snapshot selection paths/hashes
    • congested graph/cost snapshot cache hits/builds

• Minor fix

  • Fixed invalid escape regex warning in population_trips.py by using a raw string: r"\S+/public_transport/\S+".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (c8c3ac3) to head (9d1b076).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #261   +/-   ##
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  Coverage   70.42%   70.42%           
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  Files          56       56           
  Lines        2424     2424           
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  Hits         1707     1707           
  Misses        717      717           

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