Install Godot4MessagePack (pure GDScript) for MessagePack encode/decode.
Add Rust fixture generator (gen_fixtures.rs) that produces canonical
.msgpack files using rmp_serde::to_vec_named for cross-language testing.
Fixtures cover: snapshots (empty, one NPC, multi-entity with all
EntityKind variants) and player inputs (unit + data enum variants).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
