Floats before Chapter 1 (each preface's dependency map, vol4's reading-track
table) have no chapter number and printed as 0.1. The shared LaTeX header
now numbers them P.1 when the chapter counter is 0, so every volume gets it;
chapters and appendices are unchanged. Quarto offers no supported prefix for
HTML, so a post-render script (frontmatter_float_labels.py) relabels
captions, lightbox titles, and in-page references on pages under any
frontmatter/ folder to match.
Also aligns the vol2-vol4 map captions with vol1: "builds on its results".
