Harden native crypto bridge with security, correctness, and architecture improvements

[AlexNachbaur]

Critical fixes:

• Fix Android exportKey compilation error (undefined keyPair variable)
• Fix Android importKey type mismatch (KeyPair vs KeyPairEntry)
• Fix iOS silent failure in getRandomValues (now calls fatalError on CSPRNG failure)
• Fix iOS importKey key size calculation (derive from modulus, not DER blob)
• Fix iOS ASN.1 DER parsing with proper length decoding (replaces hardcoded offsets)
• Fix iOS hardcoded exponent with parsed value from key data

Security & safety:

• Eliminate all force-unwrap (as!) casts in iOS via typed KeyEntry struct
• Add synchronized blocks to all Android keyStore accesses for thread safety
• Add key usage validation in JS verify() to match WebCrypto spec
• JS getRandomValues validates returned length matches requested length

Architecture:

• Migrate bridge serialization from number[]/[NSNumber]/ReadableArray to Base64 strings across all layers (iOS, Android, TypeScript spec, JS polyfill), reducing serialization overhead from ~400% to ~33%
• Extract iOS RSA DER parsing/construction into standalone RSAKeyUtils.swift for testability
• Replace Android hand-rolled X.509 DER construction (~60 lines) with platform RSAPublicKeySpec
• Remove dead code (getCryptoAlg), fix typos, clean up unused parameters

Infrastructure:

• Fix Jest config (testMatch pattern, setupFiles path, test import paths)
• Add unit tests for digest, getRandomValues, importKey, verify, and polyfill installation
• Update mocks to match Base64 bridge interface
• Remove private:true flag, remove nonexistent ./cpp from files array
• Update Android SDK defaults to 35