[efegokdemir]

Closes #797

Summary

• Add an opt-in units::typography module with PostScript/CSS point, pica and quarter-millimetre (Q).
• Define point as 1/72 inch, pica as 12 points and Q as 1/4 mm.
• Add seven conversion assertions covering inches, millimetres and combined lengths.
• Keep typography out of the default prelude to avoid conflicting with existing uses of Q.
• Preserve the existing pc abbreviation for parsec.

Validation

• cargo run --quiet --bin numbat -- --no-config --no-init examples/tests/typography.nbt
• cargo test -p numbat
• cargo fmt --all -- --check
• git diff --check

All checks passed locally.