feat(mix): use LIONESS for Sphinx payload encryption

[chaitanyaprem]

ref PR vacp2p/nim-libp2p#2349

[Copilot]

Pull request overview

This PR migrates Sphinx payload (Delta) encryption from AES-CTR to a LIONESS wide-block construction to make payload tampering (including beyond the leading-k bytes) detectable via the existing leading-zeros integrity tag check.

Changes:

• Add a new libp2p_mix/lioness module implementing LIONESS (ChaCha20 + keyed Blake2b-256 + SHAKE128-derived round keys).
• Switch Sphinx payload encryption/decryption (including SURB/reply handling) to LIONESS while keeping header processing on AES-CTR.
• Add dedicated LIONESS test vectors/behavior tests and extend Sphinx tests with a regression test for “tamper past k bytes” detection; update callers for useSURB now returning a Result.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
libp2p_mix/sphinx.nim	Replaces Delta AES-CTR layering with LIONESS layers; updates SURB/reply encryption/decryption flow accordingly.
libp2p_mix/mix_protocol.nim	Handles useSURB failures via Result propagation/logging.
libp2p_mix/lioness.nim	New LIONESS cipher implementation + helpers (including hasLeadingZeros).
tests/test_sphinx.nim	Adds a regression test for Delta tampering past the prefix; updates SURB tests for useSURB: Result.
tests/test_lioness.nim	New sub-primitive vectors + LIONESS reference vectors and behavioral tests.

[chaitanyaprem]

Closing this PR as the new spec and the approach seems to be that we use existing cryptographic primitives sphinx already uses and also use boringssl instead of bearssl and nimcrypto for complete mix usage to keep a single dependency.