feat: add biometric key generation and signing

[LonelyCpp]

Summary

Adds on-device biometric key generation and signing to the library. The private key is generated in secure hardware (iOS Secure Enclave / Android Keystore) and can only be used after the user passes biometric authentication.

New API

createKeys(options?: { keyName?: string }): Promise<BiometricKey>      // rejects if key already exists
biometricKeysExist(options?: { keyName?: string }): Promise<boolean>
deleteKeys(options?: { keyName?: string }): Promise<boolean>            // true if deleted, false if none
createSignature(payload: string, options?: { keyName?: string; promptMessage?: string }): Promise<Signature>

• BiometricKey = { keyName, publicKey (base64), algorithm }
• Signature = { keyName, publicKey, signature (base64) }

Design notes

• RSA-2048, SHA-256 hashed payload (UTF-8 string).
• Named keys (keyName, default "default") so multiple keys can coexist — the main future-proofing decision.
• Objects returned, not primitives, so fields (algorithm, publicKey) can be added later without breaking changes.
• Options objects (not positional args) for non-breaking extension.
• Resolve/reject semantics consistent with requestBioAuth.

Changes

• src/NativeSimpleBiometrics.ts: extend TurboModule spec (codegen propagates to both platforms).
• src/index.tsx: JS wrappers with defaults + payload validation.
• android/.../SimpleBiometricsModule.kt: KeyStore + KeyGenParameterSpec; BiometricPrompt.CryptoObject for signing.
• ios/SimpleBiometrics.mm: SecKeyCreateRandomKey, SecItemCopyMatching/SecItemDelete, SecKeyCreateSignature.
• SimpleBiometrics.podspec: link Security framework.
• README + example app demo + unit tests.

Tests

• yarn typecheck, yarn lint, yarn test (7 passing) — all green.
• Codegen output verified against @react-native/codegen to confirm the generated native spec matches the Kotlin/ObjC implementations.

See docs/key-generation.md for the full implementation plan.

closes #14