fix: migrate PaperMC to v3 API, fix NeoForge beta fallback, add test suite

- Migrate Paper loader from sunset v2 API to v3 (fill.papermc.io)
- Fix NeoForge: fall back to beta versions when no stable exists (set -e + grep)
- Fix setEula/setperms ordering (was writing eula.txt before /data was writable)
- Fix setEula/serverInfoWrite to use absolute paths (/data/server/)
- Add VOLUME directive removal (was shadowing bind mounts)
- Add comprehensive API test suite (110 tests across 6 suites)
- Move test.sh to tests/integration.sh, switch to named volumes
- Add Docker build cache to CI workflows
- Add .editorconfig, expand .gitignore, improve README docs

.gitea/workflows/release.yaml

@@ -24,4 +24,6 @@ jobs:
         with:
           context: .
           push: true
-          tags: ${{ vars.REGISTRY }}/${{ gitea.repository }}:latest
+          tags: ${{ vars.REGISTRY }}/${{ gitea.repository }}:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max