Protocol.gd decodes ObserverSnapshot and PlayerInput from Rust's
rmp_serde wire format, and encodes PlayerInput for sending to server.
Handles rmp_serde enum encoding: unit variants as bare strings,
data variants as single-element maps.
8 fixture-based tests verify decode of Rust-generated fixtures,
GDScript encode/decode roundtrips, and cross-language compatibility.
All 15 tests pass (3 suites).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
