Implement switch_root functionality allowing Finit to serve as the init

in an initramfs, then transition to the real root filesystem.  Useful
for systems requiring early boot tasks like LUKS unlock, LVM activation,
or network boot before mounting the real root.

Adds INIT_CMD_SWITCH_ROOT API command, `initctl switch-root` subcommand,
and HOOK_SWITCH_ROOT plugin hook point.  The implementation gracefully
stops services, moves virtual filesystems (/dev, /proc, /sys, /run) to
the new root, deletes initramfs contents to free memory, then execs the
new init as PID 1.

See GitHub Discussion #292 for background.

doc/features.md

@@ -288,4 +288,27 @@ Rescue mode can be disabled at build time with `configure --without-rescue`.
 See the [Rescue Mode](config/rescue.md) section for more information.
 
 
+**Switch Root**
+
+Finit supports switching from an initramfs to a real root filesystem using
+the built-in `initctl switch-root` command.  This allows Finit to serve as
+the init system in an initramfs for early boot tasks (LUKS unlock, LVM
+activation, network boot) before transitioning to the real root.
+
+```bash
+# In initramfs, after mounting the real root:
+initctl switch-root /mnt/root
+```
+
+The switch-root operation:
+
+1. Runs the `HOOK_SWITCH_ROOT` hook for cleanup
+2. Stops all services gracefully
+3. Moves virtual filesystems (`/dev`, `/proc`, `/sys`, `/run`) to the new root
+4. Deletes initramfs contents to free memory
+5. Pivots to the new root and execs the new init
+
+See the [Switch Root](switchroot.md) section for complete documentation.
+
+
 [5]: https://en.wikipedia.org/wiki/Runlevel