feat: add typed request clients

[relaxfinger]

Summary

• add SharedNetworkClient for clients exposed through a typed shared instance
• bind NetworkRequest to concrete Client and Response types
• update the AppNetworkRequest pattern in the Demo and bilingual README
• update compatibility fixtures and app-layer tests for compile-time client binding

Verification

• swift test
• NetworkingKitDemo-macOS Debug build
• unsigned NetworkingKitDemo-iOS Debug build