added a lock to avoid parallel calls from different threads

AGENTS.md

@@ -30,6 +30,11 @@ Main global runtime state (in `serum-decode.cpp`):
 - Current output: `mySerum` (`Serum_Frame_Struc`).
 - Scene playback state: `sceneFrameCount`, `sceneCurrentFrame`, duration/flags/repeat, etc.
 - Identification state: `lastfound`, `lastfound_normal`, `lastfound_scene`, CRC tracking.
+- Public C API entrypoints are serialized through a recursive runtime mutex
+  because this state is process-global and scene rendering may re-enter
+  colorization internally. `Serum_Rotate()` must not run concurrently with
+  `Serum_Colorize()` while shared output buffers, flags, widths, rotation
+  tables, or scene state are being updated.
 - Scene lookup acceleration:
   - `g_serumData.frameIsScene`: frame ID -> scene/non-scene marker.
   - `g_serumData.sceneFramesBySignature`: `(mask,shape,hash)` -> matching scene frame IDs.

src/serum-decode.cpp

@@ -195,14 +195,18 @@ static void EnsureWindowsCrashHandlerInstalled() {
 }
 #endif
 
+static std::recursive_mutex g_serumApiMutex;
+
 #if defined(_WIN32) || defined(_WIN64)
-#define SERUM_API_GUARD_START(apiName)  \
-  ClearLastErrorMessage();              \
-  EnsureWindowsCrashHandlerInstalled(); \
+#define SERUM_API_GUARD_START(apiName)                                 \
+  ClearLastErrorMessage();                                             \
+  EnsureWindowsCrashHandlerInstalled();                                \
+  std::lock_guard<std::recursive_mutex> serumApiLock(g_serumApiMutex); \
   try {
 #else
-#define SERUM_API_GUARD_START(apiName) \
-  ClearLastErrorMessage();             \
+#define SERUM_API_GUARD_START(apiName)                                 \
+  ClearLastErrorMessage();                                             \
+  std::lock_guard<std::recursive_mutex> serumApiLock(g_serumApiMutex); \
   try {
 #endif
 
@@ -5269,12 +5273,14 @@ static uint32_t Serum_ColorizeWithMetadatav2Internal(uint8_t* frame,
 
 SERUM_API uint32_t
 Serum_ColorizeWithMetadatav2(uint8_t* frame, bool sceneFrameRequested = false) {
+  SERUM_API_GUARD_START("Serum_ColorizeWithMetadatav2")
   BeginProfileFrameOperation();
   const uint32_t result = Serum_ColorizeWithMetadatav2Internal(
       frame, sceneFrameRequested, IDENTIFY_NO_FRAME);
   EndProfileFrameOperation();
   MaybeLogDynamicHotPathProfileWindow(sceneFrameRequested);
   return result;
+  SERUM_API_GUARD_END("Serum_ColorizeWithMetadatav2", IDENTIFY_NO_FRAME)
 }
 
 SERUM_API uint32_t Serum_Colorize(uint8_t* frame) {