fix: ambiguous comparison between indexed accessor and cell reference

[henryiii]

🤖 AI text below 🤖

Fixes #403.

With the default storage, comparing two const accessors (as std::min_element/max_element do via libc++'s comparator) or an accessor with a cell reference failed to compile: the accessor's catch-all comparison templates and the mirrored friend operators of unlimited_storage::reference were equally good matches. The existing stdlib test did not catch this because it uses std::array<int> storage, where the cell reference is a plain int&.

• Add the missing const to the accessor-vs-accessor comparison operators, so const contexts use them instead of falling into the catch-all template.
• Constrain the generic operators in detail/operators.hpp to yield when the other operand's class handles the comparison with a member, extending the tie-breaker operator== already had (via new has_method_* detectors in detail/detect.hpp). The constraints only remove one side of a former ambiguity, so large_int and arithmetic operands are unaffected.
• Add regression tests: the reported repro plus all six operators for accessor/accessor, accessor/reference in both directions, and accessor/int. They produce 20 compile errors without the fix.

Verified locally with Apple clang (C++14/17/20/2b, libc++) and gcc-16 (C++14/20, libstdc++); full CMake test suite passes.