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

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-# AdrOS Rootfs Handoff Plan
-
-This document describes a practical roadmap to evolve AdrOS from its current permanent `initrd + overlayfs` root model toward a more conventional Unix-like boot flow where the real disk filesystem becomes `/` during normal boot.
-
-## 1. Current State
-
-Today AdrOS boots with the following model:
-
-- The kernel loads the `initrd` and initializes `fs_root` from it.
-- The kernel creates a `tmpfs` upper layer and mounts `overlayfs(initrd, tmpfs)` on `/`.
-- The kernel mounts fallback virtual filesystems such as `/tmp`, `/dev`, and `/proc`.
-- If `root=` is provided, AdrOS currently auto-detects `ext2` or `fat` and mounts the selected device on `/disk`.
-- Userspace `/sbin/init` only mounts missing virtual filesystems and parses `/etc/fstab` for extra mounts.
-- No normal boot-time `switch_root` or `pivot_root` handoff occurs.
-
-As a result, the `initrd` remains the effective permanent root filesystem during a normal boot, while the real disk root is only available as an auxiliary mount.
-
-## 2. Target State
-
-AdrOS should move toward a boot flow closer to Linux or BSD systems:
-
-- `root=` should identify the final root filesystem.
-- The real disk filesystem should become `/` during normal boot.
-- The `initrd` should be temporary in the common boot path, or reserved for rescue/special-purpose boot modes.
-- `/sbin/init` should run from the real root, not from the `initrd` overlay root.
-- `/etc/fstab` should only manage follow-up mounts after the real root is active.
-
-## 3. Recommended Direction
-
-The least disruptive path is a Linux-like model:
-
-1. Keep `initrd` support.
-2. Introduce an early userspace `/init` inside the `initrd`.
-3. Mount the real root on a staging directory such as `/newroot`.
-4. Perform `pivot_root` or `switch_root` during normal boot.
-5. Execute `/sbin/init` from the new root.
-
-This is likely easier than immediately moving to a BSD-style direct kernel `mountroot` flow because AdrOS already has `initrd`, `pivot_root`, and the basic machinery needed to mount disk filesystems before the final handoff.
-
-## 4. Implementation Milestones
-
-### Milestone A — Redefine `root=` Semantics
-
-Goal:
-- Make `root=` identify the final root filesystem rather than an automatic mount target at `/disk`.
-
-Tasks:
-- Change boot semantics so that `root=` means the intended final root device or partition.
-- Preserve existing device resolution for `/dev/hdX` and `/dev/hdXn`.
-- Mount the detected root filesystem on a staging path such as `/newroot` instead of `/disk`.
-- Keep `/disk` only as an optional secondary/manual mountpoint if it remains useful.
-
-Expected result:
-- Boot logic can resolve and stage the real root without yet replacing `/`.
-
-### Milestone B — Add Early Userspace `/init`
-
-Goal:
-- Move root handoff policy into a dedicated early boot program.
-
-Tasks:
-- Add a small `/init` program to the `initrd`.
-- Make `/init` responsible for early boot tasks related to the real root.
-- Resolve the root device passed by `root=`.
-- Mount the real root under `/newroot`.
-- Prepare required directories and prerequisites for the handoff.
-
-Expected result:
-- Early userspace is now explicitly responsible for transitioning from temporary boot environment to real system root.
-
-### Milestone C — Integrate `pivot_root` / `switch_root`
-
-Goal:
-- Make the real root become `/` in the normal boot flow.
-
-Tasks:
-- Move, recreate, or remount essential mounts such as `/dev`, `/proc`, `/tmp`, and later `/run` under the new root.
-- Perform the root handoff using `pivot_root` or a `switch_root`-style wrapper.
-- Ensure the old root is no longer the active root during normal operation.
-- `execve()` the real `/sbin/init` from the new root.
-
-Expected result:
-- AdrOS boots into the real disk filesystem as `/`.
-
-### Milestone D — Adapt `/sbin/init` and `fstab`
-
-Goal:
-- Make post-handoff userspace behavior match the new root model.
-
-Tasks:
-- Ensure `/sbin/init` assumes it is already running on the final root filesystem.
-- Keep virtual filesystem mounting logic only for missing mounts that truly belong in normal userspace init.
-- Treat `/etc/fstab` as a source of non-root follow-up mounts.
-- Avoid redoing work already completed by early `/init` or the handoff path.
-
-Expected result:
-- `/sbin/init` becomes the normal system init, not the handoff coordinator.
-
-### Milestone E — Add Boot Policy and Hardening
-
-Goal:
-- Bring the root boot path closer to established Unix-like systems.
-
-Tasks:
-- Add support for `rootfstype=`.
-- Add support for `rootflags=`.
-- Add support for `ro` / `rw` root boot modes.
-- Add support for `rootwait` and possibly `rootdelay`.
-- Later add a policy for root verification and `remount,rw` after checks.
-- Consider future support for identifiers such as `UUID=` or `LABEL=`.
-
-Expected result:
-- The root boot path becomes more robust, configurable, and closer to Linux/BSD expectations.
-
-## 5. Optional BSD-like Alternative
-
-A future alternative is a more BSD-like approach:
-
-- The kernel resolves the final root directly.
-- The kernel mounts the real root on `/` itself.
-- The kernel then starts `/sbin/init` already on the final root.
-
-This remains a valid architectural direction, but it is likely more disruptive than the Linux-like staged handoff because AdrOS already has strong building blocks for an `initrd -> /newroot -> switch_root` model.
-
-## 6. Architectural End Goal
-
-The main architectural objective is:
-
-- During normal boot, the real disk-backed filesystem becomes `/`.
-- The `initrd` stops being the effective permanent root.
-- `root=` gains the expected meaning of “final system root”.
-- `/sbin/init` executes from the final root and `/etc/fstab` handles only subsequent mounts.
-
-## 7. Summary
-
-AdrOS already has enough infrastructure to start this transition incrementally. The most practical sequence is:
-
-1. Stage the real root at `/newroot`.
-2. Add a dedicated early `/init`.
-3. Perform `pivot_root` / `switch_root` in the standard boot path.
-4. Run the final `/sbin/init` from the real root.
-5. Add boot-policy refinements such as `rootfstype`, `rootflags`, `ro/rw`, and `rootwait`.
-
-This keeps the implementation incremental while moving AdrOS toward a more conventional and scalable root filesystem boot design.