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)

export_presets.cfg

@@ -92,7 +92,7 @@ custom_template/release=""
 debug/export_console_wrapper=0
 binary_format/embed_pck=false
 texture_format/s3tc_bptc=true
-texture_format/etc2_astc=false
+texture_format/etc2_astc=true
 codesign/enable=false
 codesign/timestamp=true
 codesign/identity_type=0