docs: improve benchmark plot readability

[Ramlaoui]

Summary

• use black text for benchmark plot labels and annotations for paper readability
• keep Atompack labels visually emphasized
• exclude the internal lmdb_soa helper backend from headline story plots

Verification

• regenerated paper figures from the updated scripts
• built atompack-escience with pdflatex/bibtex/pdflatex/pdflatex

[Ramlaoui]

Closing because these plot-script changes should not be submitted to LeMaterial/atompack; the paper update lives in atompack-escience.