Prepare first downloadable release: install export templates, build 0.1.0 binaries, add publish helper

- Install Godot 4.3 export templates locally and fix macOS export
  (enable rendering/textures/vram_compression/import_etc2_astc)
- Build Linux/Windows/macOS artifacts into build/ via tools/build_releases.sh;
  Linux binary passes headless smoke test
- Add tools/publish_release.sh (creates Gitea release + uploads assets with a token)
- TODO: mark build/install steps done; publishing v0.1.0 + link verification
  remain as the single action blocker (needs GITEA_TOKEN)

project.godot

@@ -105,3 +105,4 @@ print_debug={
 
 renderer/rendering_method="gl_compatibility"
 renderer/rendering_method.mobile="gl_compatibility"
+textures/vram_compression/import_etc2_astc=true