Add crypto provider abstraction

[220nightmore-spec]

Summary

• add a pluggable CryptoProvider interface for digest/sign/verify capabilities
• add provider validation and CryptoProviderRegistry with default-provider selection and capability lookup
• add a default NodeDigestCryptoProvider for digest operations only
• export crypto provider utilities from the SDK entrypoint and document extension/security guidance in docs/CRYPTO_PROVIDER.md

This PR intentionally does not add hardware wallet support, private-key custody, or real signing implementations.

Validation

• node node_modules/jest/bin/jest.js --config jest.config.js --runTestsByPath tests/crypto/cryptoProvider.test.ts --runInBand --coverage=false
• node node_modules/eslint/bin/eslint.js src/crypto/cryptoProvider.ts src/crypto/index.ts
• node node_modules/typescript/bin/tsc --noEmit --pretty false filtered for src/crypto|tests/crypto: no crypto-related errors
• git diff --cached --check

Full repository typecheck is still blocked by existing main-branch issues unrelated to this PR, including missing @stellar/stellar-sdk/axios declarations, duplicate exports in src/index.ts, and pre-existing websocket/testing/middleware/wallet type errors.

Closes #281