Restructure to multi-module Gradle project for multi-loader support

[Copilot]

• Phase 1: Scaffold multi-module Gradle structure (buildSrc, root build.gradle, settings.gradle, gradle.properties)
• Phase 2: Create common module (build.gradle + move common-eligible sources and resources)
• Phase 3: Create platform abstraction layer (NicephoreConfigProvider interface, Services object)
• Phase 4: Create NeoForge module (build.gradle + move NeoForge-specific sources, NeoForgeConfigProvider, NicephoreNeoForge entry point)
• Phase 5: Update common source files to use Services.config instead of NicephoreConfig.Client
• Phase 6: Verify structure integrity (buildSrc compiles, no stale NeoForge imports in common, all references consistent)
• Phase 7: Run validation (code review + CodeQL passed)

Note: Full Gradle build could not be verified in sandbox due to network restrictions (maven.neoforged.net unreachable). BuildSrc compiles successfully. All source-level references verified clean. Please run ./gradlew :neoforge:build locally to confirm.