Multiloader scaffold: Architectury common+fabric+forge+neoforge (1.21.1)

Convert the single-loader NeoForge project into an Architectury multiloader:
- root/settings/subproject build.gradle, Gradle pinned to 8.10.2 (Loom needs 8.x)
- common: generic SkinsgCommands<S> + SkinsgClient bootstrap
- per-loader entrypoints (Fabric ClientModInitializer, Forge/NeoForge @Mod) + metadata
- migrate-to-multiloader.sh moves the loader/version-agnostic core into common/
- MULTILOADER.md: build/toolchain/proxy + 1.20.1 switch incl. rewritten cape mixin

main (working NeoForge 1.21.1) untouched.

common/build.gradle

@@ -0,0 +1,10 @@
+architectury {
+    common(rootProject.enabled_platforms.split(","))
+}
+
+dependencies {
+    // Fabric Loader is used at compile time only for the @Environment annotations,
+    // which Architectury remaps to the right annotation on each platform.
+    modImplementation "net.fabricmc:fabric-loader:${rootProject.fabric_loader_version}"
+    modApi "dev.architectury:architectury:${rootProject.architectury_api_version}"
+}