Stack-allocated builder for small ImmutableArray results (<= 4 items).
 Holds four inline slots and spills to an ImmutableArray.Builder on the 5th item.
 ToImmutable() dispatches to the exact-size ImmutableArray.Create overload for
 small counts, avoiding both the Builder allocation and the size-fitting copy.

 Use as a mutable local:
     let mutable xs = SmallArrayBuilder<'T>()
     xs.Add(item)
     ...
     xs.ToImmutable()

 Single-shot: do not reuse after ToImmutable().
 IsByRefLike forces the struct to stay on the stack — it can't be boxed, captured in
 a closure, stored in a heap field, or cross an async boundary. This prevents the
 silent-copy bugs typical of mutable value types.