corvus v0.1.0

corvus v0.1.0 -- first tagged release

P0 special-function surface, complete and audited: erf, erfc, erfinv,
erfcinv, lgamma, regularized incomplete gamma P/Q, regularized
incomplete beta P/Q.

• C++20, std-only public API (std::span); Highway behind an internal
  op facade with runtime multi-target dispatch.
• Per-tier ULP bounds measured on real silicon (docs/ACCURACY.md):
  AVX3_ZEN4 native + AVX2/SSE4/SSSE3/SSE2 capped (Zen 4), Linux CI
  sweep + sanitizers, NEON (macOS arm64 CI), MSVC toolchain.
• Reference oracles independently certified (verification harness with
  baked-in negative controls).
• install/export requires a system Highway (find_package(hwy >= 1.4));
  FetchContent builds are build-tree-only.
• API not yet stable; P1 families (digamma, inverse incomplete
  gamma/beta, Bessel I0/I1) ahead.