quantikz2 raw interpretation

[dginev]

This PR rescues arXiv:2508.15260v1 from a Fatal regression when using raw interpretation.

It ought to be the last PR for this iteration of improving the arXiv conversion regression guards.

Summary

• fix \hphantom display math leak in text/horizontal mode

• tcolorbox: pre-define \tcb@use@autoparskip before raw TeX loading, since pgfkeys initialization may not complete and the \AtBeginDocument hook at tcolorbox.sty would call it undefined.

• color: define \current@color, \default@color, and \reset@color with safe defaults. These DVI color macros were previously left undefined, causing errors in pgf/tikz color processing.

• latex_constructs: support optional [label] argument in \AtBeginDocument and \AtEndDocument, matching the modern LaTeX hooks system (since 2020/10/01). Without this, the [label] leaked as literal text into the document output.

Minimal test

\documentclass{article}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{quantikz2}
\definecolor{tintedcolor}{gray}{.80}%

\begin{document}
\begin{quantikz}[background color=tintedcolor]
  \meter[1]{}
\end{quantikz}
Some text with $x=1$.
\end{document}

Disclosure

Diagnostic and patch via Claude Opus 4.6 "heavy effort".

I have manually verified this patch recovers arXiv:2508.15260v1 out of regression status.

PR carried forward from the forked draft at dginev#12