docs: remove ROOTFS_HANDOFF_PLAN.md

docs: remove ROOTFS_HANDOFF_PLAN.md

All items from the document have been successfully implemented:
- Milestone A: root= semantics redefined (mounts on /newroot)
- Milestone B: early userspace /init added
- Milestone C: pivot_root/switch_root integrated
- Milestone D: /sbin/init adapted for new root model
- Milestone E: boot policy and hardening (rootfstype, rootflags, ro/rw, rootdelay, rootwait, UUID=, LABEL=, filesystem verification, remount,rw)

The boot flow now follows the Linux-like model described in the document:
- initrd is loaded and mounted as overlayfs on /
- root= mounts the real filesystem on /newroot
- Early userspace /init performs handoff via pivot_root
- /sbin/init executes from the new root
- initrd is no longer the effective permanent root

Test Results:
- Smoke test: 131/131 PASS
- Battery test: 157/157 PASS
- Analyzer: PASS
- Zero regressions