feat: replay transport for deterministic testing

[fridaypetra55-afk]

Summary

Add a replay transport that records all RPC interactions to a JSON fixture file and can replay them deterministically in tests, eliminating the need for live network calls in unit tests.

Changes

• New src/replayTransport.ts:
  • ReplayTransport.record(baseTransport, path?, options?) – wraps any RpcTransportAdapter and logs every request/response pair; call save() to write JSON fixture to disk or toJSON() for in-memory access
  • ReplayTransport.replay(fixture) – serves pre-recorded responses deterministically with per-method FIFO queues; no live network calls made
  • ReplayTransport.parse(json) – parses a fixture JSON string into a FixtureFile
  • ReplayFixtureError / ReplayFixtureParseError error classes
  • BigInt values survive serialisation round-trips via { __bigint: "..." } sentinel
• All symbols exported from src/index.ts
• 25 unit tests in test/replay_transport.test.ts

Testing

• 25 new tests pass
• Full test suite passes (pre-existing failure in issue_409_412 is unrelated)
• Lint/type check clean (pre-existing FinalizationRegistry errors in base codebase)

Closes #395

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