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.

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

@@ -0,0 +1,31 @@
+modLoader = "javafml"
+loaderVersion = "[52,)"
+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.'''
+
+[[dependencies.skinsg]]
+modId = "forge"
+type = "required"
+versionRange = "[52,)"
+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"