Generated GoogleTest stubs for Z/src.

Usage:
  python3 tests/gtest/generate_gtests.py    # generate test sources and CMakeLists
  cmake -S . -B build
  cmake --build build -- -j5
  ctest --test-dir build -j5

Notes:
- Tests are lightweight smoke stubs that compile and assert true. Replace with meaningful assertions per-function.
- Uses FetchContent to download GoogleTest at configure time.