feat(native-sdks): add BLFieldEncrypt to Swift + Kotlin platform SDKs
- Swift: BLFieldEncrypt.swift + BLFieldEncryptTests.swift (22 tests)
- CryptoKit AES-256-GCM, BLEncryptedField Codable struct
- encrypt/decrypt, AAD support, generateKey, keyFromHex, isEncrypted
- Data hex helpers (hexString, init?(hexString:))
- Kotlin: BLFieldEncrypt.kt + BLFieldEncryptTest.kt (21 tests)
- javax.crypto AES-256-GCM, BLEncryptedField data class
- encrypt/decrypt, AAD support, generateKey, keyFromHex, isEncrypted
- ByteArray/String hex extension functions
- Wire-compatible: same EncryptedField JSON structure as @bytelyst/field-encrypt (TS)
- { __encrypted: true, v: 1, alg: 'aes-256-gcm', ct, iv, tag, dekId }
- All hex-encoded, 12-byte IV, 16-byte auth tag
- Fix: ByteLystPlatform.kt getString() → read() (pre-existing compile error)
ee762b4612