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.

neoforge-26_1/build.gradle

@@ -0,0 +1,32 @@
+// Loader assumed to be NeoForge (see root plan's Open Assumptions — confirm the actual 26.1.x
+// loader ecosystem before Phase 10). Uses the modern ModDevGradle plugin; requires a recent JDK
+// (21+) unlike the two legacy leaves.
+
+plugins {
+    id 'net.neoforged.moddev' version '2.0.+'
+}
+
+sourceCompatibility = targetCompatibility = JavaVersion.VERSION_21
+
+sourceSets {
+    main {
+        java {
+            srcDir '../common/src/main/java'
+        }
+    }
+}
+
+neoForge {
+    version = project.neoforge_26_1_version
+
+    runs {
+        client {}
+        server {}
+    }
+
+    mods {
+        mcmapper {
+            sourceSet sourceSets.main
+        }
+    }
+}