Fast Snapshot Load for QEMU

QEMU's current snapshot load blocks until all guest RAM is loaded before starting execution. This project implements on-demand snapshot loading using userfaultfd - loading only device state upfront, then serving guest page faults on-demand while a background thread prefetches remaining pages. The design builds on existing postcopy migration and mapped-ram infrastructure. Deliverables include a working fast snapshot load implementation, unit tests in migration-test.c, and complete documentation.