docs(wangapayfast-rs): align README with HTTP/API capabilities

[mitch1009]

Summary

Update crate documentation to match current capabilities (including outbound API/onsite HTTP support).

Scope

• Rewrite feature overview in README.md
• Add capability matrix by feature flag
• Add examples for ITN-only mode and API client mode

Acceptance Criteria

• README no longer claims outbound HTTP is unsupported
• Feature table is accurate and complete
• Examples compile or are clearly marked pseudo-code