Add matrix TOC with cross-reference columns

Replace the default LaTeX TOC with a table matching the Carmina
Leonis style: song titles with columns for reference books (MO,
PfLB) and the current songbook page number. Uses expl3 iow to
write song data to a .songtoc file during compilation and reads
it back on the second pass. Alternating row colors via rowcolors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

songbook.tex

@@ -18,7 +18,7 @@
 \end{titlepage}
 
 % --- Table of Contents ---
-\tableofcontents
+\printsongtoc
 \clearpage
 
 % --- Songs (alphabetical) ---