perf(dbf): reduce per-value decoding overhead

[devdudumuniz]

Summary

• decode NumPy byte-string columns with a list comprehension
• centralize the optimized path in _decode_column
• use the same implementation for full-file and streaming reads

Performance

A local microbenchmark on representative fixed-width NumPy byte strings showed approximately 1.51x faster decoding at both 1,000 and 100,000 values.

Validation

• pytest -q pysus/tests/data/test_dbf_reader.py pysus/tests/data/test_dbf_reader_parity.py (40 passed)
• black --check pysus/data/dbf_reader.py
• flake8 pysus/data/dbf_reader.py

[codecov-commenter]

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Additional details and impacted files

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  Files           ?      169           
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  Hits            ?    19063           
  Misses          ?      953           
  Partials        ?        0           

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