Optimized math hot-path (deg2rad, rad2deg, arc_hav)

[gistrec]

Optimize math hot-path

Micro-optimizations in geo/detail/math.hpp — these helpers are called on every geodesic computation, so per-instruction cost matters.

deg2rad / rad2deg: extract kDegToRad / kRadToDeg constants
The expression degrees * kPi / 180.0 parses as (degrees * kPi) / 180.0. Without -ffast-math, the compiler is not allowed to fold kPi / 180.0 at compile time (IEEE 754 forbids reordering), so it emits an fdiv at runtime. A named constexpr factor produces a single fmul instead of fmul + fdiv.

arc_hav: std::clamp → std::min / std::max
std::clamp uses operator<, which on ARM64 lowers to fcmp + fcsel. std::min / std::max on double map to a single fmin / fmax instruction. Saves a couple of cycles on the hot path.

Behavior is unchanged — codegen only.