Fix output position for compiled variant and expected formatting

[fhgffy]

Compiled formatting into a char* buffer produces 42) for std::variant<int>{42} instead of variant(42). std::expected and std::unexpected have the same issue: their prefixes advance the local iterator, but the nested formatter receives the original context output position.

Advance the context to that iterator in write_escaped_alternative before calling the nested formatter. Add regression tests for all three wrappers, nested variants, monostate, returned output positions with surrounding text, and unchanged string/character escaping.

Validation on upstream main at 9197f51593dab5f637454a8889fb9638e737711a, using GCC 15.1.0 (MinGW) and CMake 4.3.2:

• All three new tests fail before the fix and pass afterward.
• C++23 Debug: complete available CTest suite passes (22/22).
• C++17 Debug: compile-test and std-test pass (2/2); C++23-only expected cases are not exercised in this configuration.
• clang-format 21.1.8 and git diff --check pass.

Linux, MSVC, and older compiler configurations have not been run locally.

[vitaut]

Thanks for the fix!