docs: honesty pass on status + platform claims

- README: "not playable yet" callout under the screenshots (no audio,
  broken front-end models/text, expect crashes past the level intro).
- README/building.md/ci.yml: platform testing is Windows-only. The Linux
  build is provided but has had zero functional testing at runtime; the
  CI job is a CMake-configure smoke test, not a Linux-support claim.

Claude-Session: https://claude.ai/code/session_01UpTM1AcBRJuTetoZNcCD5V

.github/workflows/ci.yml

@@ -12,6 +12,10 @@ on:
 # assets: the PC-port tooling, the generated ROM-asset symbol files, and a
 # CMake configure of all three regions (which checks REGION_DEFS, the source
 # list, and that SDL2 / zlib / OpenGL are discoverable).
+#
+# This is a configure-only smoke test. It does NOT compile or run the port, and
+# it running on ubuntu is NOT a statement that the port builds or works on
+# Linux — it has only ever been built and tested on Windows (MSYS2).
 
 jobs:
   validate: