hst: support ephemeral overlay mounts

This is useful for reusing a readonly template without autoroot.

Signed-off-by: Ophestra <cat@gensokyo.uk>

check/absolute.go

@@ -31,6 +31,8 @@ func (e AbsoluteError) Is(target error) bool {
 type Absolute struct{ pathname unique.Handle[string] }
 
 var (
+	_ fmt.GoStringer = new(Absolute)
+
 	_ encoding.TextAppender    = new(Absolute)
 	_ encoding.TextMarshaler   = new(Absolute)
 	_ encoding.TextUnmarshaler = new(Absolute)
@@ -40,6 +42,10 @@ var (
 	_ encoding.BinaryUnmarshaler = new(Absolute)
 )
 
+func (a *Absolute) GoString() string {
+	return fmt.Sprintf("check.MustAbs(%q)", a.String())
+}
+
 // ok returns whether [Absolute] is not the zero value.
 func (a *Absolute) ok() bool { return a != nil && *a != (Absolute{}) }