feat: C++ (RAII, header-only) bindings over the native C ABI (LEO-322) (#14)

Header-only C++17 RAII wrapper over the sw_* C ABI, mirroring the shipped Python
binding (LEO-320). Move-only Session/Snapshot handles, Error : std::exception
(noexcept, allocation-free), an enum-folded ReadingType, a Reading value type with
size()/at()/operator[]/input-iterator iteration/readings(), and a sensorwatch::hpp
INTERFACE CMake target with a dependency-free ctest. Verified on MSVC /W4 /WX (live
HWiNFO), AddressSanitizer, and MinGW gcc 15.2 (strict warnings); four external
review rounds (Copilot, codex, claude, kimi).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>