Add optimized transaction serialization utilities

[220nightmore-spec]

Summary

• add transaction serialization helpers that wrap transaction XDR + network passphrase in a versioned base64 JSON envelope
• add optional diagnostics metadata and a WeakMap-backed TransactionSerializationCache to avoid repeated toXDR() calls for the same transaction/options
• add parser-injected deserialization so protocol parsing stays caller-owned and compatible with the Stellar SDK runtime in use
• export serialization utilities from the SDK entrypoint and document usage/performance notes in docs/TRANSACTION_SERIALIZATION.md

Validation

• node node_modules/jest/bin/jest.js --config jest.config.js --runTestsByPath tests/serialization/transactionSerialization.test.ts --runInBand --coverage=false
• node node_modules/eslint/bin/eslint.js src/serialization/transactionSerialization.ts src/serialization/index.ts
• node node_modules/typescript/bin/tsc --noEmit --pretty false filtered for src/serialization|tests/serialization: no serialization-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 #277