Phase 0: scaffold Gradle multi-project mod skeleton

common/ (shared source) plus three leaf modules: forge-1_12_2 (primary,
Enigmatica 2 target) and forge-1_7_10 both build via anatawa12's
Gradle-7-compatible ForgeGradle 1.2/2.3 forks (verified: both leaves build
clean on Gradle 7.6 / JDK 8). neoforge-26_1 is structurally scaffolded but
excluded from the default build pending its own toolchain in Phase 10.

.gitignore

@@ -0,0 +1,24 @@
+# Gradle
+.gradle/
+build/
+out/
+
+# Per-leaf Forge/NeoForge run dirs
+run/
+runs/
+logs/
+
+# IDE
+.idea/
+*.iml
+.vscode/
+.classpath
+.project
+.settings/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Local overrides
+local.properties