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/build.gradle

@@ -0,0 +1,41 @@
+plugins {
+    id "com.gradleup.shadow"
+}
+
+architectury {
+    platformSetupLoomIde()
+    neoForge()
+}
+
+configurations {
+    common
+    shadowBundle
+    compileClasspath.extendsFrom common
+    runtimeClasspath.extendsFrom common
+    developmentNeoForge.extendsFrom common
+}
+
+dependencies {
+    neoForge "net.neoforged:neoforge:${rootProject.neoforge_version}"
+    modApi "dev.architectury:architectury-neoforge:${rootProject.architectury_api_version}"
+
+    common(project(path: ":common", configuration: "namedElements")) { transitive false }
+    shadowBundle project(path: ":common", configuration: "transformProductionNeoForge")
+}
+
+processResources {
+    inputs.property "version", project.version
+    filesMatching("META-INF/neoforge.mods.toml") {
+        expand version: project.version
+    }
+}
+
+shadowJar {
+    configurations = [project.configurations.shadowBundle]
+    archiveClassifier = "dev-shadow"
+}
+
+remapJar {
+    input.set shadowJar.archiveFile
+    archiveClassifier = null
+}