Share LaTeX token and mode boundaries

[Happypig375]

Closes #296

Summary

• add one internal UTF-16 source-span tokenizer for control words/symbols, groups, whitespace/raw text, and $/$$ boundaries
• make both the math command dictionary and text parser consume the shared command boundary
• use one production mode-transition contract for $, $$, \(/\), and \[/\]
• preserve the control-sequence and whitespace behavior merged in Respect TeX control-sequence boundaries #276, including \@ as a control symbol
• keep all public parser and painter entry points unchanged; global settings and the later parser migration remain outside this PR

Validation

• CSharpMath.Core.AtomTests.LaTeXParserTest: 319 passed
• CSharpMath.Rendering.Text.Tests.TextLaTeXParserTests: 252 passed
• scoped format verification and git diff --check: passed
• independent architecture review: approved

The focused tests cover every token kind and exact UTF-16 span, control-word termination, escaped symbols/braces, nested groups, all text/math delimiter forms, exact malformed/unclosed-mode positions, and independent recovery after a failed parse.