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.

migrate-to-multiloader.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# One-time restructure of the single-loader NeoForge tree into the Architectury
+# multiloader layout. Run once from the repo root on the `multiloader` branch:
+#   bash migrate-to-multiloader.sh
+# It only MOVES the loader/version-agnostic code (config, gif, client, mixins)
+# into common/, and the NeoForge metadata into neoforge/. The changed files
+# (generic commands, per-loader entrypoints, mod metadata) are added separately.
+set -euo pipefail
+cd "$(dirname "$0")"
+
+CJ=common/src/main/java/xyz/nightsync/skinsg
+CR=common/src/main/resources
+mkdir -p "$CJ" "$CR" neoforge/src/main/resources/META-INF
+
+# --- loader/version-agnostic core -> common ---
+git mv src/main/java/xyz/nightsync/skinsg/SkinsgConfig.java "$CJ"/
+git mv src/main/java/xyz/nightsync/skinsg/gif            "$CJ"/
+git mv src/main/java/xyz/nightsync/skinsg/client         "$CJ"/
+git mv src/main/java/xyz/nightsync/skinsg/mixin          "$CJ"/
+git mv src/main/resources/skinsg.mixins.json            "$CR"/
+
+# --- superseded / rewritten: generic commands, per-loader entrypoints + fresh metadata ---
+git rm src/main/java/xyz/nightsync/skinsg/commands/SkinsgCommands.java
+git rm src/main/java/xyz/nightsync/skinsg/SkinsgMod.java
+git rm src/main/resources/META-INF/neoforge.mods.toml
+
+# clean up now-empty old source roots
+rm -rf src
+
+echo "Migration done. Now add the new files (SkinsgCommands generic, SkinsgClient,"
+echo "per-loader entrypoints + metadata) — see MULTILOADER.md — then ./gradlew build."