feat: support integer long division

[Happypig375]

Summary

• add semantic LongDivision and step records for bounded nonnegative integer division
• parse canonical \longdiv{numerator}{denominator} in CTAN argument order with deterministic source-positioned errors
• lay out the quotient, divisor bracket, underlined dividend, aligned subtraction work, and one final remainder
• serialize and clone/equality round-trip the semantic atom
• enforce a 28-digit input limit and reject zero denominators and malformed operands

Validation

• LongDivisionTests: 22 passed
• backend rendering/scaling tests: 14 passed
• existing Skia and Avalonia golden suites: MathDisplay 96/96 and MathInline 96/96 per backend
• new display and inline golden fixtures cover remainder/multi-row, internal-zero, exact, and short/zero-quotient division
• malformed operand tests assert exact Unicode caret positions and error details
• Release builds, scoped format checks, and git diff checks passed

Closes #302