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.

common/README.md

@@ -0,0 +1,19 @@
+# common
+
+Loader-agnostic Java: protocol types, config model, and the interfaces each leaf module
+(`forge-1_7_10`, `forge-1_12_2`, `neoforge-26_1`) implements against its own Minecraft/Forge
+API generation.
+
+This module is **not** consumed as a compiled binary dependency. The legacy leaves (1.7.10,
+1.12.2) target Java 8; `neoforge-26_1` targets a modern JDK. To avoid cross-version binary
+compatibility issues, each leaf module adds `common/src/main/java` directly to its own source
+set (see the `sourceSets.main.java.srcDirs` line in each leaf's `build.gradle`) and compiles it
+itself, once per leaf, against its own toolchain. Keep this module free of any
+Minecraft/Forge/NeoForge API usage — it must compile standalone under plain `javac`.
+
+## Layout
+
+- `protocol/` — wire types shared with the backend (delta events, chat/waypoint payloads, link
+  request/response). Mirrors the WS protocol described in the root plan.
+- `config/` — the common config model (backend URL, server token, tracking/reconciliation
+  intervals) each leaf loads via its own loader-specific config system.