Add TeX/LaTeX equation rendering

[tiwa-cc]

Add TeX/LaTeX equation support for scientific and technical DocIR documents.

This issue focuses on rendering mathematical expressions safely and consistently across supported renderers.

Goal

• Add an equation-oriented DocIR block
• Support TeX/LaTeX source as structured content
• Keep rendering behavior renderer-managed
• Avoid allowing arbitrary HTML inside equations
• Preserve meaning when exporting to Markdown

Candidate block shape

* type: equation
  title: Mass-energy equivalence
  tex: |
  E = mc^2
  display: block
  variables:

  * symbol: E
    description: energy
    unit: J
  * symbol: m
    description: mass
    unit: kg
  * symbol: c
    description: speed of light
    unit: m/s

Renderer behavior

• Plain HTML renderer should support readable equation output
• Bootstrap renderer should render equations with suitable spacing and styling
• Markdown renderer should export equations as TeX-compatible Markdown
• TeX rendering should be optional and should not be required for normal documents

Possible rendering engines

• KaTeX
• MathJax

Configuration direction

engine = "katex"
mode = "cdn"

Possible modes

• cdn
• bundled
• plain
• pre_rendered

Acceptance criteria

• An equation or math block schema is defined
• TeX content is escaped and handled safely
• Plain HTML and Bootstrap renderers can render or gracefully fallback
• Markdown renderer exports TeX in a readable form
• At least one sample document includes an equation
• Invalid or unsupported math rendering modes produce readable errors

Non-goals

• Do not implement a full TeX engine
• Do not allow arbitrary HTML inside math blocks
• Do not require math rendering for documents that do not use math blocks
• Do not make KaTeX or MathJax mandatory for all renderers