Use cc (Apple Clang) on macOS, gcc on Linux — unified CI/local parity

- Add scripts/env.sh: detects true hardware arch (even under Rosetta),
  verifies compiler supports target arch, adds arch prefix on macOS
- macOS uses cc/c++ everywhere (tests + builds) — ASan works natively
- Linux uses gcc/g++ everywhere — full sanitizer support
- Remove broken CC ?= gcc-14 from Makefile (GNU Make built-in overrides it)
- Scripts accept --arch flag and CC=/CXX= overrides