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.

gradle.properties

@@ -1,39 +1,32 @@
 # Sets default memory used for gradle commands. Can be overridden by user or command line properties.
-org.gradle.jvmargs=-Xmx1G
-org.gradle.daemon=true
+org.gradle.jvmargs=-Xmx3G
+org.gradle.daemon=false
 org.gradle.parallel=true
-org.gradle.caching=true
-org.gradle.configuration-cache=true
 
-#read more on this at https://github.com/neoforged/NeoGradle/blob/NG_7.0/README.md#apply-parchment-mappings
-# you can also find the latest versions at: https://parchmentmc.org/docs/getting-started
+# --- Mod ---
+mod_id=skinsg
+mod_name=HD Skin Support (Gravit)
+mod_version=0.1.0
+mod_group_id=xyz.nightsync.skinsg
+mod_license=All Rights Reserved
+mod_author=Nghtly
+
+# --- Stage 1 target: Minecraft 1.21.1 (Architectury multiloader) ---
+minecraft_version=1.21.1
+minecraft_version_range=[1.21.1]
+parchment_minecraft=1.21.1
+parchment_version=2024.11.17
+
+# Loaders / APIs for 1.21.1
+architectury_api_version=13.0.8
+fabric_loader_version=0.16.9
+fabric_api_version=0.115.0+1.21.1
+forge_version=1.21.1-52.0.63
+neoforge_version=21.1.219
+
+enabled_platforms=fabric,forge,neoforge
+
+# NeoGradle parchment (kept for reference; loom uses parchment_* above)
 neogradle.subsystems.parchment.minecraftVersion=1.21.1
 neogradle.subsystems.parchment.mappingsVersion=2024.11.17
-# Environment Properties
-# You can find the latest versions here: https://projects.neoforged.net/neoforged/neoforge
-# The Minecraft version must agree with the Neo version to get a valid artifact
-minecraft_version=1.21.1
-# The Minecraft version range can use any release version of Minecraft as bounds.
-# Snapshots, pre-releases, and release candidates are not guaranteed to sort properly
-# as they do not follow standard versioning conventions.
-minecraft_version_range=[1.21.1]
-# The Neo version must agree with the Minecraft version to get a valid artifact
-neo_version=21.1.219
-# The loader version range can only use the major version of FML as bounds
 loader_version_range=[1,)
-
-## Mod Properties
-
-# The unique mod identifier for the mod. Must be lowercase in English locale. Must fit the regex [a-z][a-z0-9_]{1,63}
-# Must match the String constant located in the main mod class annotated with @Mod.
-mod_id=skinsg
-# The human-readable display name for the mod.
-mod_name=HD Skin Support (Gravit)
-# The license of the mod. Review your options at https://choosealicense.com/. All Rights Reserved is the default.
-mod_license=All Rights Reserved
-# The mod version. See https://semver.org/
-mod_version=0.1.0
-# The group ID for the mod. It is only important when publishing as an artifact to a Maven repository.
-# This should match the base package used for the mod sources.
-# See https://maven.apache.org/guides/mini/guide-naming-conventions.html
-mod_group_id=xyz.nightsync.skinsg