Add modern API based on C++26 features

Start by reviewing the existing public program_options API and the proposed V2 API, including the listed C++26 features and backward-compatibility goal. The issue names no files or tests; the work would be complete only when the V2 API is designed, the implementation is modernized, and the new public API is exposed.

ITNOA

There have been significant changes from C++03 to C++26 that fundamentally affect API design in C++.

Consequently, I propose designing a new API for program_options based on C++26 best practices.

Key modern C++ features we can leverage include:

• Modules: For faster build times and cleaner library exposure.
• Concepts: For improved template metaprogramming and clearer constraints.
• Contracts: For more robust code and runtime/compile-time assertions.
• std::optional: For a more declarative API when handling optional values.
• std::variant & std::any: For type-safe, flexible option storage.
• Structured Bindings: For a cleaner, more intuitive API when unpacking values.
• std::span & std::string_view: For zero-copy, high-performance string and array views.
• std::expected: For modern, expressive error handling instead of exceptions or return codes.
• Compile-Time Reflection: For smarter, automatic type inference and a significantly cleaner API footprint.

To achieve this, we should redesign the public API in a C++26-native way and modernize the underlying codebase.

To make this actionable, I suggest the following roadmap:

• Design the V2 API: Create a new, modern API namespace/module separate from the legacy API to maintain backward compatibility.
• Modernize the Codebase: Migrate the internal implementation details to C++26.
• Expose the C++26 API: Provide the new public-facing API for modern users.