Use AES for a closed Rijndael scope with its existing 128-bit block size

Replace a parameterless RijndaelManaged using resource with Aes.Create(), explicitly retaining FeedbackSize128. Require a closed local with only forwarded key/IV/mode/padding/feedback properties and transform/key-generation calls. Reject every BlockSize reference, escapes, captures, identity inspection, initialization and conditional snapshots. Preserve caller algorithms, key size, mode, padding, argument evaluation and disposal. Opt-in modern-.NET compatibility port.