Parse inline math spans inside text commands

[Happypig375]

Summary

• let math-mode \text{...} temporarily re-enter normal inline math for balanced $...$ spans
• preserve Roman text around the span, keep \$ literal, and restore parser mode/font state on every success and failure path
• reject display-style $$/$$$ inside \text with an exact outer-source position instead of silently accepting an empty span
• serialize literal dollar atoms safely without changing raw-dollar behavior outside text mode

This is intentionally a focused fix independent of the broader parser-unification work in #58.

Coverage

• exact Collatz cases reproduction from the issue, including both \text{if $n$ ...} branches
• semantic parse → serialize → parse equality with commands, braces, scripts, and escaped dollars
• exact unmatched/display-delimiter source indices and parser-state restoration after failure
• committed Skia golden showing Roman words around math-italic nested variables

Validation

• Core and Rendering Release builds: passed (0 warnings/errors)
• complete LaTeXParserTest: 295/295 passed
• complete SkiaSharp MathDisplay: 93/93 passed
• scoped formatting verification: passed
• git diff --check: passed
• senior review: approved
• no public API changes

Fixes #205