Build: verify native-port setup on Linux and Windows

[akratch]

Problem

The native port is expected to build on Linux and Windows because it is portable C + SDL2 + OpenGL, but macOS/arm64 is the most exercised maintainer path. Public launch should make platform support precise and evidence-backed without relying on hosted GitHub Actions.

Desired end state

• docs/BUILDING.md has verified Linux and Windows dependency/setup notes.
• Linux CMake configure/build/test is covered by local preflight evidence and contributor smoke reports.
• At least one contributor has reported a full native build and boot smoke on Linux.
• Windows/MSYS2 setup is either verified or honestly marked less exercised with exact failure notes.

Validation

• cmake -B build -DCMAKE_BUILD_TYPE=Release
• cmake --build build -j
• ctest --test-dir build --output-on-failure
• ./tools/validate_quick.sh, when a local ROM is available
• ./scripts/ci/check_release_ready.sh

Asset rule

Do not attach ROMs, extracted assets, save files, or screenshots containing game imagery.

[jsmolina]

right now the windows build is giving black screen (well adding some strange garbled graphics), but audio.

my linux r36s gives black screen and choppy audio (even at 4096). Device is weak, that's why a port like the perfect dark made sense