refactor(headers)!: enforce canonical domain include paths

[LimiNode]

Summary

• Prepare the version-aware Doxygen/Pages publication workflow and validate generated documentation on pull requests.
• Align the generated site layout with the Doxygen Awesome pattern used by log-it-cpp, including a wider sidebar and working light/dark theme toggle.
• Make pkg-config relocatable and validate standard/multiarch installs.
• Organize the public C++ headers by domain and remove obsolete root-level forwarding headers.
• Add an explicit migration note for selective includes and enforce the canonical root/domain inventory in CMake.
• Prepare the intentional source-breaking release as v2.0.0.

Breaking header-layout change

The full-library include remains unchanged:

#include <time_shield.hpp>

Selective includes must use canonical domain paths, for example:

time_shield/validation.hpp       -> time_shield/core/validation.hpp
time_shield/time_conversions.hpp -> time_shield/conversions.hpp
time_shield/DateTime.hpp          -> time_shield/date_time.hpp
time_shield/ntp_client.hpp        -> time_shield/ntp/ntp_client.hpp

Verification

• MinGW Debug CTest: 63/63 passed.
• MinGW Release CTest: 63/63 passed.
• Header include-policy and root/domain inventory checks passed.
• Installed consumer passes with NTP enabled and disabled.
• Doxygen 1.12 generation and Playwright desktop/mobile layout smoke tests pass.
• vcpkg CI pins the overlay port to the checked-out PR revision and computes its SHA512 dynamically.