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
• render the header through a private display that sizes the right parenthesis with existing math-table logic, positions it independently with a half-rule overlap, and draws one bar across the delimiter and dividend
• keep right-aligned place-value subtraction work and exactly one final remainder
• serialize and clone the semantic atom, enforce a 28-digit limit, and reject zero denominators or malformed operands

Validation

• LongDivisionTests: 27 passed
• dedicated backend, scaling, and pixel-connectivity tests: 16 passed
• Skia and Avalonia MathDisplay and MathInline golden runs: 97/97 per selector
• the official CTAN longdiv.tex was compiled with Tectonic 0.17.0 and rasterized at 1200 DPI; its overbar and raised parenthesis form one connected pixel component
• all 10 final display/inline long-division goldens were visually inspected and verified with a 4-neighbor component check; Text and Script styles also verify the junction in both backends
• changed-file formatting, git diff checks, and a scoped secret-pattern scan passed

Review

• independent Codex review found that the first candidate lacked a direct junction regression; the new Skia/Avalonia Text/Script pixel test was added
• follow-up review caught a transient equality/hash mismatch in the private header atom; those overrides were removed
• final independent re-review approved the current diff with no unresolved findings
• human review remains required because this changes core rendering

Closes #302