ε - ChaCha20-Poly1305 AEAD + HKDF crypto upgrade + C3PO rewrite + docs
Crypto:
- Replace broken ChaCha20 (static nonce) with ChaCha20-Poly1305 AEAD
- HKDF-SHA256 key derivation from per-device factory NVS master keys
- Random 12-byte nonce per message (ESP32 hardware RNG)
- crypto_init/encrypt/decrypt API with mbedtls legacy (ESP-IDF v5.3.2)
- Custom partition table with factory NVS (fctry at 0x10000)
Firmware:
- crypto.c full rewrite, messages.c device_id prefix + AEAD encrypt
- crypto_init() at boot with esp_restart() on failure
- Fix command_t initializations across all modules (sub/help fields)
- Clean CMakeLists dependencies for ESP-IDF v5.3.2
C3PO (C2):
- Rename tools/c2 + tools/c3po -> tools/C3PO
- Per-device CryptoContext with HKDF key derivation
- KeyStore (keys.json) for master key management
- Transport parses device_id:base64(...) wire format
Tools:
- New tools/provisioning/provision.py for factory NVS key generation
- Updated flasher with mbedtls config for v5.3.2
Docs:
- Update all READMEs for new crypto, C3PO paths, provisioning
- Update roadmap, architecture diagrams, security sections
- Update CONTRIBUTING.md project structure
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