Add cache-first route policy provider

[relaxfinger]

Summary

• add optional URLRouterPolicyProvider product that depends on URLRouter
• add cache-first bootstrap, TTL refresh, stale-cache fallback, file and in-memory caches, and extensible source/payload validation interfaces
• integrate the provider in the demo and document the recommended App refresh lifecycle in English and Chinese
• expand CI to build the provider for tvOS and watchOS

Validation

• route contract validation
• swift test (15 tests)
• provider builds for macOS, tvOS 17 simulator, and watchOS 10 simulator
• feature package builds and URLRouterDemo iOS Simulator build