Add configurable precision for complex stringification

Description

Add an opt-in StringMaker<std::complex<T>> with configurable precision for float, double and long double components. Each component type has its own setting, defaulting to max_digits10.

This follows the existing optional stringmaker mechanism, including the ALL toggle, CMake option, generated configuration header and Bazel template. It retains the standard (real,imag) stream format and leaves scalar precision settings and the non-opt-in fallback unchanged.

Validation

• 11 new cases / 37 assertions cover round-tripping, runtime precision, special values, independent settings and stream reuse.
• GCC 14.2 / C++14: basic preset 81/81 CTest checks and full all-tests preset 143/143 pass. The discovery checks use a private writable runtime/temp directory.
• Clang 19.1.7 / C++20 shared library with the individual option: 81/81 checks pass.
• Installed-package consumers verify cross-translation-unit settings and actual assertion diagnostics; the generated amalgamation also compiles and runs the API.
• With ALL stringmakers enabled, the existing pair/tuple reporter-baseline mismatch remains: 80/81 checks pass, and all eight differing output files are byte-identical on pristine base and candidate. No reporter baselines were changed.

GitHub Issues

Refs #2876.