Deprecate ZapClient toward NostrZapClient

[AustinKelsay]

Part of #154
Blocked by: #156

Goal

Make NostrZapClient the canonical Public Facade. Mark ZapClient as a Compatibility Alias with @deprecated, re-export both only through the nip57 barrel (root imports from that barrel), and update NIP-57 README guidance.

Acceptance

• ZapClient carries @deprecated pointing to NostrZapClient.
• Root exports NostrZapClient/ZapClient via ./nip57 (not a deep ./nip57/client path).
• README documents the canonical facade.
• Existing NIP-57 client tests remain green for both classes.

Notes

AFK. Do not remove ZapClient in 0.x.

[AustinKelsay]

Implemented on feature/cleanup-tier-ab-api-surface and included in PR #163 targeting staging. Spec #154.