Preserve motion queue ABI for feed override

wasm-core/include/sim_motion_queue.h

@@ -30,10 +30,11 @@ typedef struct sim_motion_command {
     sim_posemath_segment segment;
     uint32_t id;
     uint32_t tag;
-    uint32_t flags;
 } sim_motion_command;
 
-#define SIM_MOTION_COMMAND_FLAG_RAPID 1u
+/* The high tag bit is reserved for Rapid blocks so the command ABI remains
+   compatible with existing checkpoints and WASM clients. */
+#define SIM_MOTION_COMMAND_TAG_RAPID (UINT32_C(1) << 31)
 
 typedef struct sim_motion_state {
     sim_posemath_pose position;