[Polish] CI/CD: GitHub Actions builds PC + WebGL on push

[sja-thedude]

Goal\nNo manual builds.\n\n## Tasks\n- game-ci/unity-builder workflow\n- Cache Library/ between runs\n- Upload artifacts as release assets

---

Part of the NEXUS: The Sentient Galaxy initiative under PlayPath. Project lives at /projects/nexus-sentient-galaxy/.