rootfs-handoff: Milestone A - redefine root= semantics to mount on /newroot

rootfs-handoff: Milestone A - redefine root= semantics to mount on /newroot

- Create /newroot directory during kernel init
- Mount root= device on /newroot instead of /disk
- Keep /disk as optional secondary mountpoint for manual use
- Update comments to reflect staging purpose of /newroot
- This change prepares for future pivot_root handoff

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