feat(offline): add route-scoped runtime provider

[rdlabo]

Summary

• add an opt-in route-scoped offline provider and explicit single-flight initializer
• isolate route runtime services and optional configuration tokens from any parent offline runtime
• preserve the existing root provideOffline API and initialization behavior
• document the parent-ready-guard / child-authorization-guard pattern

Compatibility

This is additive. Existing applications can continue using provideOffline without migration. This PR does not change package versions.

Verification

• ng test kit --watch=false (53 files, 967 tests)
• ng lint kit
• npm run prebuild:kit
• consuming SeatKeep integration: 116 app files / 866 tests, site test, lint, format check, and CI production build
• root + route coexistence DI regression coverage
• diff limited to the provider, its spec, and the offline guide