feat: exact rational DMS parsing and Degrees canonicalization

[yelliver]

Summary

• parseDMS: numeric DMS no longer collapses to floating-point degrees; it returns a reduced exact rational (3600·d + 60·m + s) / 3600 wrapped in Degrees.
• Degrees canonical: exact conversion to radians now applies to any rational degree (e.g. 19/2°), not only integers.
• DMS() canonical: builds degrees via ce.number([totalSec, 3600]) instead of a float sum, preserving exactness through the boxing pipeline.
• Tests: updated raw-parse expectations to use reduced rationals; evaluation tests assert exact LaTeX via simplify().latex instead of floating-point approximations.

Motivation

DMS angles are naturally exact rationals of a degree: 9°30' is 19/2°, not 9.5 as a float. The old pipeline aggregated components with floating-point arithmetic at parse time (deg + min/60 + sec/3600), which introduced rounding error before canonicalization.

That mattered downstream: Degrees only had an exact rational→radian path for integer degrees. A parsed angle like 5°37'30" became a float, missed the exact branch, and expressions such as converting to radians and simplifying could not reduce to clean fractions (e.g. π/32).

This change keeps DMS exact end-to-end - parse as reduced rationals, box DMS() the same way, and extend Degrees canonicalization to all rational arguments - so degree-to-radian conversion stays symbolically correct.