v3.1.0

Added

• Precise integer time (opt-in utl/time_point.hpp): TimeDuration and
  TimeStamp, nanosecond-native int64 wrappers over Time<int64_t> with
  affine semantics — TimeStamp - TimeStamp yields a TimeDuration,
  TimeStamp +/- TimeDuration yields a TimeStamp, and TimeStamp + TimeStamp
  is deleted. to_time() bridges to the dimensional Time<double> (seconds).
  The floating-point unit machinery is untouched.
• std::chrono interop (opt-in utl/chrono.hpp): to_duration / to_timestamp
  convert from std::chrono::duration / time_point, and overloads of
  to_chrono convert TimeDuration / TimeStamp back. Kept separate so
  <chrono> is only included on demand.
• Readable dimension-mismatch diagnostics: adding, subtracting or comparing
  quantities of different dimensions now fails with a clear static_assert
  message (e.g. "utl: cannot add quantities of different dimensions") instead of
  an overload-resolution wall of "no matching operator" notes.
• Single-header amalgamation: single_include/utl/utl.hpp, generated from the
  multi-header tree by tools/amalgamate.py and kept in sync by CI, for
  zero-setup drop-in use. (The opt-in utl/io.hpp is not amalgamated.)
• Five more coherent derived units, each with a from-constituents constructor
  and an automatic result-type mapping: Vergence (1/m, diopter),
  Density (kg/m³), CurrentDensity (A/m²), Concentration (mol/m³) and
  Luminance (cd/m²). Luminance (cd/m²) is distinct from Illuminance
  (lm/m²) thanks to the angle dimension.