fix(ci): address PR #31 review — harden cross-encoder fixture pipeline

- Fail on encode errors instead of silently writing empty .msgpack files
- Fail test on missing/empty fixture dir instead of silent skip
- Add all missing action variants (MoveSouth, MoveEast, MoveWest,
  Unpause, ToggleStanceDown, WalkAway) to GDScript fixture generator
- Add GDScript fixture staleness check to make pre-pr
- Validate repo root detection before writing outside client/
- Add file.flush() before close in headless mode
- Document fixture failure recovery in DEVOPS.md

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

server/tests/serialization.rs

@@ -1013,15 +1013,14 @@ fn malformed_input_in_batch_rejects_entire_batch() {
 /// (runs client/tests/gen_client_fixtures.gd via Godot headless)
 #[test]
 fn gdscript_generated_fixtures_deserialize() {
+    // CWD is server/ when cargo test runs (Cargo sets it to the package root)
     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;
-    }
+    assert!(
+        fixture_dir.exists(),
+        "GDScript fixture directory not found at {}. Run `make fixtures-client` to generate.",
+        fixture_dir.display()
+    );
 
     let mut count = 0;
     for entry in fs::read_dir(&fixture_dir).expect("read gdscript fixture dir") {
@@ -1088,13 +1087,11 @@ fn gdscript_generated_fixtures_deserialize() {
         count += 1;
     }
 
-    if count == 0 {
-        eprintln!(
-            "SKIP: no .msgpack files found in {}. Run `make fixtures-client` to generate.",
-            fixture_dir.display()
-        );
-        return;
-    }
+    assert!(
+        count > 0,
+        "No .msgpack files found in {}. Run `make fixtures-client` to generate.",
+        fixture_dir.display()
+    );
     eprintln!("Verified {} GDScript-generated fixtures", count);
 }