Debug: add LaTeX numbering check assertions

[rbeezer]

Summary

• Add a debug mechanism (-x debug.numbering.check yes) that embeds PreTeXt-computed numbers into LaTeX source and compares them against LaTeX's own counter values at compile time
• Separate macros for block elements (\ptxnumbercheck via \thetcbcounter) and divisions (\ptxdivisionnumbercheck via \csname the#2\endcsname)
• When the flag is false, no macros are defined and nothing is emitted — zero cost

Developing the check mechanism revealed a cross-format formatting bug: subnumbered figure-like elements inside sidebyside panels produced 6.1.(a) instead of 6.1(a). The period separator is now suppressed for these elements. This fix affects all output formats (LaTeX, HTML, etc.) and is a separate commit from the debug infrastructure.

Test results

Configuration	Block checks	Division checks	Mismatches
Sample article	474	237	14 block (all explained)
Sample book — no parts	175	105	0
Sample book — decorative parts	175	105	0
Sample book — structural parts	175	105	0

The 14 article mismatches: 12 from a missing structure-number template for figure/sbsgroup/sidebyside/figure, 1 from openproblem placeholder numbers, 1 from a figure inside decorative exercises. None are introduced by this branch — they are pre-existing bugs surfaced by the check.

Test plan

• Build sample article and sample book with -x debug.numbering.check yes, confirm warnings match documented mismatches
• Build without the flag, confirm no new macros or assertions in output
• Verify HTML output only differs in subnumber formatting (6.1(a) vs 6.1.(a))

Claude Opus 4.6, acting as a coding assistant for Rob Beezer

[rbeezer]

• Before and after HTML looks good, other than the formatting of subnumbered items.
• Before and after LaTeX is no change.
• Before and after LaTeX, with new debugging enabled, looks as expected.
• PDF compilation, with debugging assertions, indicates two minor bugs that need further investigation.

Note: if the debug flag is not set, nothing is added to the LaTeX source, no macros, no checks.

Code itself looks good.