[cpp] Fix test expected output to include refString fields

Updates test.sh expected output to match current SkeletonSerializer format.
Adds test infrastructure documentation to AGENTS.md.

AGENTS.md

@@ -41,3 +41,19 @@ Use lowercase prefixes exactly as listed below.
 - If multiple runtimes are changed, include multiple prefixes.
 - If all runtimes are changed, use `[runtimes]`.
 - If shared files at repo root are changed, include the runtime prefix(es) impacted by that change.
+
+## spine-cpp / spine-libgdx test infrastructure
+
+The spine-cpp headless test (`spine-cpp/tests/test.sh`) validates that spine-cpp produces the same parsed skeleton data as spine-libgdx. Both runtimes have a `SkeletonSerializer` that serializes loaded skeleton data to JSON for comparison.
+
+Files:
+- `spine-cpp/tests/HeadlessTest.cpp` - C++ test that loads skeleton and serializes to JSON
+- `spine-cpp/tests/SkeletonSerializer.h` - C++ JSON serializer for skeleton data
+- `spine-libgdx/spine-libgdx-tests/src/com/esotericsoftware/spine/HeadlessTest.java` - Java equivalent
+- `spine-libgdx/spine-libgdx-tests/src/com/esotericsoftware/spine/utils/SkeletonSerializer.java` - Java equivalent
+
+When tests disagree:
+1. Check if the skeleton was loaded from JSON or binary. Run both to isolate which parser differs.
+2. Compare the SkeletonSerializer implementations. They must output identical JSON structure.
+3. Check for unported changes in spine-libgdx (SkeletonBinary.java, SkeletonJson.java) and port them to spine-cpp.
+4. Update `spine-cpp/tests/test.sh` expected output if the serializer format changed intentionally.

spine-cpp/tests/test.sh

@@ -23,14 +23,14 @@ SPINEBOY_ANIM="idle"
 # Expected output pattern - first 10 lines of skeleton JSON data
 EXPECTED_OUTPUT="=== SKELETON DATA ===
 {
+  \"refString\": \"<SkeletonData-spineboy-pro>\",
   \"type\": \"SkeletonData\",
   \"bones\": [{
+      \"refString\": \"<BoneData-root>\",
       \"type\": \"BoneData\",
       \"index\": 0,
       \"parent\": null,
-      \"length\": 0,
+      \"length\": 0,"
-      \"color\": {
-        \"r\": 0.607843,"
 
 log_title "Spine-C++ Test"
 log_detail "Platform: $(uname)"