Announced layoffs threatening to decimate a weakened US Education Department are stoking fears among special education advocates, families, and school leaders over the potential fallout for more than 190,000 students with disabilities in Massachusetts. The federal cuts, which were announced Oct. 10 amid a government shutdown, would largely eliminate Education Department staff who ensure students receive support from their schools through the Individuals with Disabilities Education Act (IDEA), the 50-year-old landmark law that guarantees that students with disabilities have a right to a free and appropriate public education.