* build(cxx): align all builds on a C++14 baseline
CMake, Make, tests, tools, and examples now require C++14 so every supported build path shares the same minimum language level. The compiler checks and user-facing documentation reflect that baseline.
* fix(test): keep cores scoped to the failing test
Clear prior cores before each test binary. A passing death test can leave a core that would otherwise be used to diagnose an unrelated later failure.
* revert src/butil/type_traits.h
* build(cmake): keep modern CMake policy compatibility
Raise the minimum CMake version range to 3.16...3.28 so CMake 4.x no longer treats the project as relying on removed pre-3.5 policy compatibility. The range preserves a conservative 3.16 runtime floor while documenting policy validation through 3.28.
* build(cmake): modernize target-scoped CMake configuration
Move shared compile definitions, include paths, options, and link dependencies onto interface targets so top-level, src, tools, tests, and standalone examples consume consistent build settings without relying on global directory state.
Examples now share a small CMake helper, and the gtest download path is updated to configure with modern CMake while keeping downloaded gtest headers ahead of Homebrew's C++17-only installation.
Bright Chen