Introduces FeedEngine that consumes FeedSource instances and manages
the dependency graph for context flow and item collection.
- Validates dependency graph (missing deps, circular references)
- Topologically sorts sources for execution order
- Runs fetchContext() in dependency order, accumulating context
- Runs fetchItems() on all sources with final context
- Supports reactive updates via onContextUpdate/onItemsUpdate
- Graceful error handling (continues after source failures)
Marks DataSource, ContextProvider, ContextBridge, Reconciler, and
FeedController as deprecated in favor of FeedSource + FeedEngine.
Co-authored-by: Ona <no-reply@ona.com>
