Phase 3 — Wallet Integration

BIP39 seed phrase (12–24 words) implemented from scratch with official test vectors. Shamir's Secret Sharing (M-of-N, GF(256)) from scratch. ChaCha20-Poly1305 AEAD wallet encryption with PBKDF2-HMAC-SHA512 KDF (210,000 iterations). Atomic wallet writes. New binary wallet format (FERR v2) with CSV migration. Change address via dedicated derivation path. UTXO selection sorted descending by value. Four new RPC handlers: getwalletinfo, encryptwallet, importseed, getshamirshares. HTTP Basic Auth via .rpc.cookie.