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.