Add CRYPTOGRAPHY.md — crypto primer for contributors

[Copilot]

Contributors with general programming backgrounds have no single reference explaining the cryptographic concepts and algorithms LoraMesh depends on, making it hard to reason about protocol decisions or safely contribute to crypto-adjacent code.

Changes

• docs/CRYPTOGRAPHY.md (new, ~520 lines) — programmer-oriented guide covering:

  • Core concepts: symmetric/asymmetric encryption, AEAD, KDFs, digital signatures, ECDH, forward secrecy — each with a minimal code snippet grounded in the actual codebase
  • Every primitive in use (Ed25519, X25519, ChaCha20-Poly1305, XChaCha20-Poly1305, BLAKE2b, Argon2id) with rationale for choosing it over the common alternative
  • Protocol walkthroughs: Noise_IK handshake (step-by-step with diagram) and Double Ratchet (symmetric + DH ratchet, including LoRa out-of-order handling)
  • End-to-end message lifecycle tracing key setup → encryption → decryption
  • Common pitfalls table (nonce reuse, timing side-channels, weak passphrases, key residency)
  • Curated further reading per topic

• README.md — added CRYPTOGRAPHY.md to the repository structure table

Original prompt

I need to know more about cryptography. I know the basics as a programmer but that's it

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