feat: support LaTeX middle delimiters

[Happypig375]

Closes #303

Summary

• parse and round-trip LaTeX \middle delimiters, including multiple, nested, composite, and null boundaries
• model inner atoms as immutable ordered segments plus middle boundaries while preserving the legacy single-list API
• typeset all outer and middle delimiters to one shared extent and retain delimiter assemblies/order
• make multi-segment inners explicitly atomic in editor navigation while preserving legacy no-middle descent
• add source-aware errors, public API ledger entries, structural equality/hash behavior, and cross-backend coverage

Validation

• middle parser/model cases: 11 passed
• typesetter geometry: 3 passed (Display/Text/Script)
• SkiaSharp + Avalonia rendering checks: 6 passed
• editor regression selectors: 1,014 passed
• MathList/MathAtom selectors: 25 passed
• Release Core/Rendering builds passed; scoped format and diff checks passed
• independent validation and review: approved