fix: render text macros in pspicture labels

[pyramation]

Summary

\rput(1.5,1){\LaTeX} rendered the literal string \LaTeX instead of the logo — visible on mathapedia's How it works diagram, which the old site rendered correctly. Labels were stored as raw source (text: m[3]) and handed to MathJax verbatim; MathJax only touches delimited math, so a bare \LaTeX survives as text. Prose doesn't have the problem because the parser runs the Text transforms over it, and those map \LaTeX → $\LaTeX$.

So labels now get the same Text pass as prose — not a \LaTeX special case, so {\bf ...}, ''``, --- etc. work in labels too.

The catch is that a label often is math, and the transforms must not touch it: -- inside $...$ means two minus signs, and rewriting it to – feeds MathJax broken input (there's a test pinning exactly that, from when this last regressed). parseLabel therefore splits on $...$ spans and transforms only the text between them, reassembling math byte-for-byte:

{\LaTeX\ vs $a--b$}   ->  {$\LaTeX$ vs $a--b$}
{$a--b$}              ->  {$a--b$}          (unchanged)
{\LaTeX}              ->  {$\LaTeX$}

parseText (prose) is unchanged — it keeps calling the transforms over the whole line, now via the extracted parseTextTransforms. rputgroup (an rput whose contents are shapes, not a label) is untouched.

Verified by rendering mathapedia against these builds: the diagram label now matches the old site pixel-for-pixel. Full suite (591 tests, 10 projects) and the workspace build pass; the tracked browser bundle is regenerated.

Link to Devin session: https://app.devin.ai/sessions/e51e8ecb94d241bd8d89e65e642c3e43
Requested by: @pyramation

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