feat(ci): add make fixtures-client target for GDScript->Rust cross-encoder validation (#475)

Closes the bidirectional protocol compatibility loop (D-030 Layer 1):
- GDScript fixture generator (20 fixtures: inputs, boundary ticks, batch)
- Rust decoder test verifying all GDScript-encoded fixtures deserialize
- Makefile target with generation + verification in one step

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/tests/fixtures/gdscript/boundary_tick_127.msgpack

@@ -0,0 +1 @@
+うtickｦaction･Pause

server/tests/fixtures/gdscript/boundary_tick_128.msgpack

@@ -0,0 +1 @@
+うtickﾌ€ｦaction･Pause

server/tests/fixtures/gdscript/boundary_tick_2147483647.msgpack

@@ -0,0 +1 @@
+‚¤tickÒÿÿÿ¦action¥Pause

server/tests/fixtures/gdscript/boundary_tick_255.msgpack

@@ -0,0 +1 @@
+うtickﾌ�ｦaction･Pause

server/tests/fixtures/gdscript/boundary_tick_32767.msgpack

@@ -0,0 +1 @@
+うtickﾑ�ｦaction･Pause

server/tests/fixtures/gdscript/boundary_tick_65535.msgpack

@@ -0,0 +1 @@
+‚¤tickÍÿÿ¦action¥Pause

server/tests/fixtures/gdscript/input_interact.msgpack

@@ -0,0 +1 @@
+うtickdｦaction→Interactげtarget_entity_idﾀ､verbﾀ

server/tests/fixtures/gdscript/input_move_north.msgpack

@@ -0,0 +1 @@
+うtickdｦactionｩMoveNorth

server/tests/fixtures/gdscript/input_move_northeast.msgpack

@@ -0,0 +1 @@
+うtickdｦactionｭMoveNortheast

server/tests/fixtures/gdscript/input_move_northwest.msgpack

@@ -0,0 +1 @@
+うtickdｦactionｭMoveNorthwest

server/tests/fixtures/gdscript/input_move_southeast.msgpack

@@ -0,0 +1 @@
+うtickdｦactionｭMoveSoutheast

server/tests/fixtures/gdscript/input_move_southwest.msgpack

@@ -0,0 +1 @@
+うtickdｦactionｭMoveSouthwest

server/tests/fixtures/gdscript/input_pause.msgpack

@@ -0,0 +1 @@
+うtickdｦaction･Pause

server/tests/fixtures/gdscript/input_perception_mode.msgpack

@@ -0,0 +1 @@
+うtickﾌﾈｦaction�UsePerceptionModeｧthermal

server/tests/fixtures/gdscript/input_toggle_stance_up.msgpack

@@ -0,0 +1 @@
+うtickdｦactionｮToggleStanceUp

server/tests/serialization.rs

@@ -1004,6 +1004,100 @@ fn malformed_input_in_batch_rejects_entire_batch() {
     );
 }
 
+/// GDScript-generated fixtures must deserialize correctly (D-030 Layer 1, #475).
+/// Validates the reverse direction: GDScript encoder -> Rust decoder.
+/// Together with all_fixtures_deserialize (Rust -> GDScript), this closes the
+/// cross-encoder compatibility loop.
+///
+/// Fixtures generated by: make fixtures-client
+/// (runs client/tests/gen_client_fixtures.gd via Godot headless)
+#[test]
+fn gdscript_generated_fixtures_deserialize() {
+    let fixture_dir = std::path::Path::new("tests/fixtures/gdscript");
+
+    if !fixture_dir.exists() {
+        eprintln!(
+            "SKIP: GDScript fixtures not found at {}. Run `make fixtures-client` to generate.",
+            fixture_dir.display()
+        );
+        return;
+    }
+
+    let mut count = 0;
+    for entry in fs::read_dir(&fixture_dir).expect("read gdscript fixture dir") {
+        let entry = entry.expect("read dir entry");
+        let path = entry.path();
+        if path.extension().and_then(|e| e.to_str()) != Some("msgpack") {
+            continue;
+        }
+        let name = path.file_stem().unwrap().to_str().unwrap().to_string();
+        let bytes = fs::read(&path).unwrap_or_else(|_| panic!("read fixture {}", name));
+
+        if name.starts_with("input_batch") {
+            let inputs: Vec<PlayerInput> = rmp_serde::from_slice(&bytes).unwrap_or_else(|e| {
+                panic!("deserialize GDScript batch fixture {}: {}", name, e)
+            });
+            assert!(
+                !inputs.is_empty(),
+                "batch fixture {} should not be empty",
+                name
+            );
+        } else if name.starts_with("input_") || name.starts_with("boundary_tick_") {
+            let input: PlayerInput = rmp_serde::from_slice(&bytes).unwrap_or_else(|e| {
+                panic!("deserialize GDScript input fixture {}: {}", name, e)
+            });
+            // Verify specific fixtures for extra confidence
+            match name.as_str() {
+                "input_move_north" => {
+                    assert_eq!(input.tick, 100);
+                    assert!(matches!(input.action, PlayerAction::MoveNorth));
+                }
+                "input_perception_mode" => {
+                    assert_eq!(input.tick, 200);
+                    assert!(
+                        matches!(input.action, PlayerAction::UsePerceptionMode(ref s) if s == "thermal")
+                    );
+                }
+                "input_interact" => {
+                    assert_eq!(input.tick, 100);
+                    assert!(matches!(input.action, PlayerAction::Interact { .. }));
+                }
+                "boundary_tick_256" => {
+                    assert_eq!(input.tick, 256, "int_16 asymmetry: tick=256");
+                }
+                "boundary_tick_32767" => {
+                    assert_eq!(input.tick, 32767, "int_16 asymmetry: tick=32767");
+                }
+                "boundary_tick_65536" => {
+                    assert_eq!(input.tick, 65536, "int_32 asymmetry: tick=65536");
+                }
+                "boundary_tick_2147483647" => {
+                    assert_eq!(
+                        input.tick, 2147483647,
+                        "int_32 asymmetry: tick=2^31-1"
+                    );
+                }
+                _ => {} // Other fixtures: deserialization success is sufficient
+            }
+        } else {
+            panic!(
+                "unknown GDScript fixture naming convention: {} — add a deserialization branch",
+                name
+            );
+        }
+        count += 1;
+    }
+
+    if count == 0 {
+        eprintln!(
+            "SKIP: no .msgpack files found in {}. Run `make fixtures-client` to generate.",
+            fixture_dir.display()
+        );
+        return;
+    }
+    eprintln!("Verified {} GDScript-generated fixtures", count);
+}
+
 /// NearbyInteraction.object_type round-trips through MessagePack (#422).
 /// Verifies object_type=Some(Container) survives the wire.
 #[test]