Use cryptographic zeroization for inferred local byte vectors

Replace exact Array.Clear(local, 0, local.Length) when a var local's array creation proves one-dimensional byte storage. Retain both reads, null Length failure, finally placement and pin cleanup order. Scope closure/ref/unsafe/async/iterator guards to the containing method; skip unknown initializers, other ranks/elements, fields, parameters, slices, named arguments, internal comments and conditional snapshots. Existing explicit-vector rule retains its global bounds. Opt-in compatibility port.