What

Document the dual-build strategy around the optional C++20 modules support (requires CMake 3.28+), so downstream consumers know when and how to enable it.

• Current: Module support exists (CMakeLists.txt:176-250) but selection criteria are not documented
• Expected: A short guide covering prerequisites, toolchain matrix, and fallback behaviour
• Scope: docs/guides/CXX20_MODULES.md (new), linked from README.md

Why

• C++20 modules are still experimental in some compilers; silently enabling them can confuse consumers
• Dual-build expectations (header-only fallback vs. modules) should be explicit

How

Technical Approach

1. Create docs/guides/CXX20_MODULES.md with:
   • Compiler/CMake matrix
   • -DCOMMON_BUILD_MODULES=ON usage
   • Known limitations (e.g. import under sanitizers)
2. Link from README.md under "Advanced usage"

Acceptance Criteria

• Guide page exists and is referenced from README
• Matrix covers GCC 14+, Clang 17+, MSVC 2022+, Apple Clang 15+
• Fallback behaviour explained