Add peptide entrapment benchmark and FDP plots

[jspaezp]

Summary

• Generate one shuffled entrapment peptide per eligible tryptic target, with a FDRBench-compatible pair file and FASTA.
• Build/search the paired library with timsseek, then calculate lower-bound, combined, and paired precursor FDP from results.parquet.
• Write TSV, PNG, and PDF reports; support plotting an existing TSV without rerunning the search.
• Document the workflow and generation/prediction defaults.

Checks

• task fmt
• uv run --group dev ruff check bench/entrapment.py bench/test_entrapment.py
• uv run --group dev python -m pytest bench/test_entrapment.py -q (5 passed)
• Generated plots from a 786,201-row entrapment report.