Port to NeoForge 1.21.1

[exocyt0sis]

This PR ports Immersive Weathering to NeoForge on Minecraft 1.21.1.

Summary:

• migrates the project to the NeoForge 1.21.1 toolchain while preserving the published jar name
• updates dependency metadata and README notes for Moonlight Lib 1.21.1-3.0.5
• replaces legacy networking hooks with NeoForge payload registration
• updates resource and data paths for 1.21.x conventions
• adds startup validation with clear English errors for incompatible NeoForge, Minecraft, or Moonlight versions
• keeps the packaged icon and validates expected metadata during the jar build

Validation:

• gradlew build completed successfully on the ported branch
• the branch merges cleanly into 1.20.0-multiloader with no conflicts

[AstralOrdana]

no fuckin way

[exocyt0sis]

Sorry? 🙂