Streaming AEAD does not authenticate stream structure

Summary

In all published versions of `dcrypt-symmetric` before 2.0.0, version-1 GCM and ChaCha20-Poly1305 streams used unauthenticated terminator, counter, and length fields. Decryptors trusted transmitted counters, allocated from unbounded lengths, and discarded plaintext that did not fit a caller's single read buffer. An attacker could obtain successful truncated EOF, omit, replay, or reorder frames within a stream, or request an allocation approaching 4 GiB.

Version 2.0.0 introduces the `DCRSTRM2` format, which authenticates its version, algorithm, random stream identifier, base nonce, strict sequence, bounded lengths, final flag, and caller AAD. It enforces authenticated finality and physical EOF, bounds frames to 16 KiB before allocation, retains partial-read plaintext, and rejects legacy version-1 input. Existing version-1 ciphertext cannot retrospectively prove completeness or ordering and must be migrated only through an application-specific, explicitly trusted process.