v2.0.0

Fixed

• Header-defined physical constants violated the ODR — any project
  including utl/utl.hpp from two translation units failed to link.
• virtual on BaseUnit::inv() added a vtable pointer to every unit
  (sizeof(Length<float>) was 16 instead of 4).
• Mass / Acceleration returned Force; the physically correct
  Mass * Acceleration is now derived from the dimension system.
• FAMTO prefix typo (now FEMTO), DAYS_IN_YEAR was 356,
  truncated km-to-mile factor replaced by the exact mile definition.
• M_PI (POSIX) replaced with portable utl::PI; the library now
  builds warning-free under MSVC /W4.

Changed (breaking)

• Cross-unit arithmetic is derived from dimension exponents via
  UnitMapper and requires #include <utl/utl.hpp>; single-unit
  headers no longer provide it. Hand-written per-class operators were
  removed (~800 lines).
• TimeTemperatureUnit renamed to TimeUnit, ElectricChangeUnit
  renamed to ElectricChargeUnit.
• type() on dimensionless results no longer yields Angle.
• Physical constants are constexpr (immutable) instead of mutable
  globals.

Added

• Whole library is constexpr; unit math evaluates at compile time.
• Unit literals: auto d = 120.0_km + 500.0_m;
• Comparison operators, compound assignment, unary minus.
• Stream output via utl/io.hpp: 30 [s^-1 m].
• Dimension-aware sqrt, pow<N>, abs, min, max.
• Imperial/engineering conversions: yd/ft/in, lb/oz, day,
  Wh/cal, hp, bar/atm/psi, mph.
• CMake utl::utl INTERFACE target with install/find_package
  support.
• CI on gcc, clang, MSVC and ASan/UBSan, with warnings as errors.
• mdBook user guide deployed to GitHub Pages.