Refine dataset section language

[aadarwal]

Summary

• replace the Section V opening and dataset subsection with the approved language
• revise the Fig. 12 caption to describe the L0–L4 progression without assigning an unverified physical interpretation to the Bragg-cavity panel
• retain the approved dataset release ending unchanged
• update the compiled paper asset

Validation

• compiled arxiv/main.tex with Tectonic
• checked the compile log for unresolved citations, unresolved references, LaTeX errors, and overfull boxes
• rendered and visually inspected manuscript pages 11 and 12
• confirmed that citation [39] still resolves to the frozen PixCell Dataset release
• confirmed that only arxiv/main.tex and assets/pixcell-paper.pdf are included