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

.gitignore

@@ -1 +1,17 @@
-data/
+# Runtime data
+data/
+
+# Editor/IDE
+*.swp
+*.swo
+*~
+.vscode/
+.idea/
+*.iml
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Test artifacts
+*.log