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.

build.gradle

@@ -0,0 +1,14 @@
+// Root project intentionally has no plugins applied — each leaf module (forge-1_7_10,
+// forge-1_12_2, neoforge-26_1) uses a different loader-tooling generation and configures
+// itself independently. `common` is shared as source, not as a compiled artifact, since
+// its consumers target different Java versions (Java 8 for the legacy Forge leaves, a
+// modern JDK for neoforge-26_1) — see common/README.md.
+
+allprojects {
+    group = 'com.octoturge.mcmapper'
+    version = '0.1.0-SNAPSHOT'
+
+    repositories {
+        mavenCentral()
+    }
+}