fix: CreateUser uses wrong /etc path for composefs-native installs

[castrojo]

Summary

CreateUser in internal/post/user.go uses root = sysroot for composefs-native deployments, but sysroot/etc/ is a ghost btrfs directory — writes to it are silently discarded after first boot.

Root cause

// user.go:30
if isComposeFsNative(sysroot) {
    root = sysroot  // WRONG: sysroot/etc is not writable after boot
} else {
    deployDir, _ := DeploymentDirFn(sysroot)
    root = deployDir  // correct for ostree
}

On composefs-native systems:

• The writable /etc is bind-mounted from state/deploy/<HASH>/etc/
• sysroot/etc/ is an orphaned btrfs directory that nothing mounts
• useradd --root sysroot writes to sysroot/etc/passwd etc., which is silently discarded after first boot
• The user appears created during install but is gone on first login

Compare with WriteHostname and AppendFstabEntry which both correctly use ComposeFsDeployEtcDirFn(target) for composefs paths.

Current exposure

Zero — all production images with needs_user_creation=True use grub2/ostree path. Only systemd-boot images (Dakota) use composefs-native, and Dakota has needs_user_creation=False, so CreateUser is never called for composefs images.

But this will break silently if a future image uses both composefs-native and user creation.

Fix

if isComposeFsNative(sysroot) {
    var err error
    etcDir, err := ComposeFsDeployEtcDirFn(sysroot)
    if err != nil {
        return fmt.Errorf("finding composefs deploy etc: %w", err)
    }
    root = filepath.Dir(etcDir)  // parent of etc dir = deployment root
} else {
    deployDir, err := DeploymentDirFn(sysroot)
    ...
}

Also: the staterootHome path ostree/deploy/default/var/home should use ComposeFsVarDir(sysroot) for composefs-native.

Assisted-by: Claude Sonnet 4.5 via pi