Add integration test coverage for Esplora sync/apply/persist flow

[reez]

We need integration tests for real Esplora sync flows.

Scope:

• create or load wallet
• sync via EsploraClient
• apply update to wallet
• persist state
• assert post-sync invariants

Acceptance criteria:

• Integration test exists for Esplora sync/apply/persist.
• Test can run in CI, or CI strategy is documented.
• Failures provide useful diagnostics.
• Requirements for production-ready release #16 links this issue.

Notes:

• Keep tests deterministic where possible.
• Document timeouts and retry behavior if network is used.