[FEATURE] Add Salsa20 Stream Cipher Module

[Aakif-Kohari]

Summary

CryptoViz has no ARX-based stream cipher in the visualizer despite @noble/ciphers already being a project dependency (per README tech stack) and supporting Salsa20 natively. Adding it demonstrates a modern stream-cipher construction alongside the existing XOR/OTP stream examples.

What's missing

• No lib/cipher/salsa20.ts
• No visualizer entry, worker routing, or tests

Proposed implementation

• Thin instrumented wrapper around @noble/ciphers/salsa20 for real output (correctness/security), with a parallel pure-TS quarter-round instrumentation path for step visualization (same "instrumented vs fast path" split used for AES per CIPHER_ENGINE.md section 5.6)
• Visualize the 4x4 state matrix and quarter-round diffusion (ADD/XOR/ROTATE) similar to how AES steps show the 4x4 state
• Security status: secure
• Verify against the official Salsa20 test vectors (256-bit key, ECRYPT test set)

Acceptance criteria

• Output matches @noble/ciphers reference output exactly
• Quarter-round steps rendered as milestone steps
• Unit tests against ECRYPT Salsa20 vectors
• Fuzz test: encrypt→decrypt round-trip for arbitrary byte arrays
• Dark/light mode support via existing theme system

Assignment

I'd like to work on this issue under ECSoC26. Will open a PR only after approval/assignment by @csxark or another maintainer.

[github-actions]

🎉 Issue Assigned

Hi @Aakif-Kohari, I'm assigning this initiative to you.

Looking forward to your contributions!

Please do the following before coding:

• ✅ Sync your fork with the latest main
• ✅ Create a new feature branch
• ✅ Implement the requested changes
• ✅ Test your work
• ✅ Open a PR referencing this issue

Wishing you the very best, and happy contributing! 🚀🚀