refactor: manual mode from absolute setpoint to velocity control, remove auto-enable at motor init

Manual mode now receives angular velocity (degrees/s) via 0xA6 frame instead of
absolute angle reference. The firmware integrates velocity * dt into gimbal angle ref.
Motors are no longer auto-enabled at DMMotorInit — they stay disabled until a mode
switch out of DEBUG. Extract ROBOT_CMD_DELAY macro to replace magic number.
Fix "volity" typo → "velocity" and update docs/protocol comments accordingly.

application/robot_task.h

@@ -113,6 +113,6 @@ __attribute__((noreturn)) void StartROBOTTASK(void *argument)
         robot_dt = DWT_GetTimeline_ms() - robot_start;
         if (robot_dt > 5)
             LOGERROR("[freeRTOS] ROBOT core Task DELAY dt=%.3f", robot_dt);
-        osDelay(2);
+        osDelay(ROBOT_CMD_DELAY);
     }
 }