refactor: compile v2 resources from KRT collections

[EItanya]

Summary

• replace the v2 Reader abstraction with typed KRT collections
• use informer-backed collections in production and krttest mocks for fixed test inputs
• retain KRT dependency tracking across ModelConfig, compiler, and harness lookups
• use direct krt.FetchOne calls with caller-owned missing-resource errors

Testing

• go test ./core/v2/...