Add resumable execution to the screening framework (#33)

[galjos]

Resolves the one MVP requirement still missing from the screening workflow (#33): resume/checkpoint (input/output formats, result collection, and per-molecule error isolation were already done).

What changed

• Incremental writes — screen() now writes <out>.json/.csv after every molecule, so an interrupted run leaves a durable partial result set (previously written only once at the end).
• screen(resume=True) — reuses the status="ok" records from a prior <out>.json (_load_completed), skips those molecules, and (re)runs the missing/previously-failed ones. A missing or corrupt prior file just runs everything.
• CLI — thermo screen --resume.

Tests

• resume skips completed molecules and re-runs previously-failed ones (only mol_b re-run after mol_a succeeded, mol_b failed);
• results written incrementally ([1, 2, 3] after each of 3 molecules);
• _load_completed tolerates missing/corrupt prior files;
• CLI --resume parsing.

Closes #33. Full suite green (patch fully covered).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.73%. Comparing base (5a7aa2a) to head (f0df9f7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   96.68%   96.73%   +0.05%     
==========================================
  Files          24       24              
  Lines        1538     1564      +26     
==========================================
+ Hits         1487     1513      +26     
  Misses         51       51              

Flag	Coverage Δ
unittests	96.73% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.