Migrate to C++20 for fbitset compatibility

[Copilot]

The fbitset dependency migrated to C++20, replacing GCC builtins with std::bit library functions (std::countl_zero, std::countr_zero, std::popcount). This updates libparenth accordingly.

Changes

• Build configuration: C++20 standard, CMake 3.12+ requirement, CMAKE_CXX_STANDARD_REQUIRED ON
• fbitset submodule: Updated to commit 765b2f7 (C++20 version)
• Code quality: Added [[nodiscard]] to API functions (opt(), n_dims(), n_factors(), get_tot())
• Documentation: Clarified find_last() behavior with unsigned overflow semantics

Compatibility Note

The updated fbitset find_last() returns SIZE_MAX (not -1) when no bits are set. Existing code handles this correctly via well-defined unsigned wrap-around:

auto top_idx = bsums.curr_sums.find_last();  // SIZE_MAX when count() == 0
auto next_idx = top_idx + 1;                  // Wraps to 0 (well-defined)

No algorithmic changes required. All tests pass.

Original prompt

The dependency fbitset has been updated to use C++20 features. Check compatibility and update libparenth. You may want to rethink d0afb59 and how to properly fix previous compatibility issues. Update to C++20, and review and improve the codebase, without compromising performance and correctness.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull Request Overview

This PR migrates libparenth to C++20 to maintain compatibility with the updated fbitset dependency, which now uses std::bit library functions instead of GCC builtins.

Key Changes:

• Updated build configuration to C++20 standard with CMake 3.15+ requirement
• Updated fbitset submodule to C++20-compatible version (commit 765b2f7)
• Added [[nodiscard]] attributes to query functions for better code safety

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
CMakeLists.txt	Updated CMake minimum version to 3.15, set C++20 standard, and enforced standard requirement
.clang-format	Updated standard specification from Cpp11 to c++20
include/libparenth.hpp	Added [[nodiscard]] attributes to API functions and documented unsigned overflow behavior in find_last() usage
deps/fbitset	Updated submodule to C++20-compatible version (765b2f7)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.