Fix deprecated df.append() and missing test prefix

[awanawana]

Summary

Fixes two issues found during code review:

1. Replace deprecated df.append() with efficient list-based approach (parsing_keystats.py)

• Issue: df.append() is deprecated in pandas 2.0+ and has O(n²) complexity
• Fix: Use list to collect rows, then create DataFrame once at the end - O(n) complexity
• Impact: Significant performance improvement when parsing large datasets

2. Fix missing test prefix (tests/test_datasets.py)

• Issue: Function def_keystats_dimensions() lacks test_ prefix, so pytest never runs it
• Fix: Renamed to test_keystats_dimensions()
• Impact: Enables an important test that validates keystats.csv dimensions

Both changes maintain backward compatibility and pass existing tests.

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Submitted by awanawana via OpenClaw agent