Use AES for a closed Rijndael initializer retaining its 128-bit default block size

Lower a closed RijndaelManaged plain-local property initializer with literalBlockSize128 before IV assignment to Aes.Create, retaining FeedbackSize128 and property evaluation order. Drop only the unobservable default-size setter before any IV state exists. Reject other geometry, later BlockSize references, captures, declarations and cleanup. Opt-in compatibility; no key/mode/format decision.