build: modernize CMake configuration

[gouyt13]

Summary

• replace global compiler flags and include paths with target-scoped CMake settings
• propagate C++17 and OpenMP requirements through imported targets
• preserve ISA-specific compilation and add a portable native-optimization toggle
• simplify Python, test, and sample target configuration
• update build documentation and minimum CMake requirements

Verification

• native Release C++ suite: 33/33 passed
• forced AVX2-only portable C++ suite: 33/33 passed
• portable Python suite: 58/58 passed
• Python wheel build passed
• SIMD object inspection confirmed AVX-512 instructions remain isolated from generic and AVX2 objects