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.

neoforge/src/main/java/xyz/nightsync/skinsg/neoforge/SkinsgNeoForge.java

@@ -0,0 +1,29 @@
+package xyz.nightsync.skinsg.neoforge;
+
+import net.minecraft.commands.CommandSourceStack;
+import net.minecraft.network.chat.Component;
+import net.neoforged.api.distmarker.Dist;
+import net.neoforged.bus.api.IEventBus;
+import net.neoforged.fml.common.Mod;
+import net.neoforged.fml.loading.FMLPaths;
+import net.neoforged.neoforge.client.event.ClientTickEvent;
+import net.neoforged.neoforge.client.event.RegisterClientCommandsEvent;
+import net.neoforged.neoforge.common.NeoForge;
+import xyz.nightsync.skinsg.SkinsgClient;
+import xyz.nightsync.skinsg.commands.SkinsgCommands;
+
+@Mod(value = "skinsg", dist = Dist.CLIENT)
+public final class SkinsgNeoForge {
+
+    private static final SkinsgCommands.Feedback<CommandSourceStack> FEEDBACK = new SkinsgCommands.Feedback<>() {
+        @Override public void success(CommandSourceStack s, String m) { s.sendSuccess(() -> Component.literal(m), false); }
+        @Override public void failure(CommandSourceStack s, String m) { s.sendFailure(Component.literal(m)); }
+    };
+
+    public SkinsgNeoForge(IEventBus modBus) {
+        SkinsgClient.init(FMLPaths.CONFIGDIR.get());
+        NeoForge.EVENT_BUS.addListener((RegisterClientCommandsEvent e) ->
+                e.getDispatcher().register(SkinsgCommands.build(SkinsgClient.config(), FEEDBACK)));
+        NeoForge.EVENT_BUS.addListener((ClientTickEvent.Post e) -> SkinsgClient.tick());
+    }
+}

neoforge/src/main/resources/META-INF/neoforge.mods.toml

@@ -0,0 +1,34 @@
+modLoader = "javafml"
+loaderVersion = "[1,)"
+license = "All Rights Reserved"
+
+[[mods]]
+modId = "skinsg"
+version = "${version}"
+displayName = "HD Skin Support (Gravit)"
+authors = "Nghtly"
+description = '''Client-side HD skins (up to 2048) and animated GIF cape support.'''
+
+[[mixins]]
+config = "skinsg.mixins.json"
+
+[[dependencies.skinsg]]
+modId = "neoforge"
+type = "required"
+versionRange = "[21.1,)"
+ordering = "NONE"
+side = "CLIENT"
+
+[[dependencies.skinsg]]
+modId = "minecraft"
+type = "required"
+versionRange = "[1.21,1.22)"
+ordering = "NONE"
+side = "CLIENT"
+
+[[dependencies.skinsg]]
+modId = "architectury"
+type = "required"
+versionRange = "[13.0.8,)"
+ordering = "AFTER"
+side = "CLIENT"