macOS (Apple Clang/libc++) precompiled-wheel build fails with an ambiguous
operator- — the generic operator-/operator+ templates in
MV_Operators.h were unconstrained and therefore matched arbitrary types via
argument-dependent lookup. When std::vector<TMultivector>::erase (used by
the new Join) computed __position - cbegin() under libc++, both the
standard iterator operator- and Tan::GA::operator- were viable, producing
a compile error on macos-14-arm64. The operators are now SFINAE-constrained
to true multivector types (those exposing the TValue and TBlade nested
types), so the standard iterator operator- is selected unambiguously and the
macOS wheel build compiles again.