Initial release of biometric_vault: encrypted storage for small secrets (tokens, passwords, key material) with optional biometric protection.

Features #

• Named encrypted stores with a four-call lifecycle: canAuthenticate(), getStorage(), then read() / write() / delete().
• Android 7.0+ (API 24): AES-GCM key in the Android Keystore (StrongBox when available, TEE fallback), gated by BiometricPrompt with Class 3 biometrics and optional device-credential fallback. All keystore and file work runs off the main thread, and writes are atomic.
• iOS 13+ / macOS 10.15+: Keychain items protected by SecAccessControl (Face ID / Touch ID / Apple Watch, optional passcode fallback), shipped as a Swift Package Manager package with a CocoaPods fallback. Keychain operations run off the main thread.
• Linux (libsecret), Windows (Credential Manager), and web (localStorage, not secure) backends so cross-platform code keeps working without authentication support.
• Typed, sealed error model: every failure is a subtype of the sealed BiometricVaultException (AuthException with 14 dedicated AuthExceptionCode values, StorageInvalidatedException with a StorageInvalidatedReason, or BiometricVaultPluginException), so a single switch handles all outcomes exhaustively and raw PlatformExceptions never escape.
• Precise capability reporting: canAuthenticate() distinguishes biometry lockout (errorLockedOut) and pending security updates (errorSecurityUpdateRequired) from generic hardware unavailability.
• StorageFileInitOptions with explicit per-platform durations (androidAuthenticationValidityDuration, darwinTouchIDAuthenticationAllowableReuseDuration, darwinTouchIDAuthenticationForceReuseContextDuration) and per-platform biometricOnly flags. Invalid store names or option combinations throw ArgumentError at getStorage instead of failing natively at first use.
• Localizable prompt strings via PromptInfo (AndroidPromptInfo and DarwinPromptInfo).