feat(core): add FeedController orchestration layer

Adds orchestration for feed reconciliation with context-driven updates:

- FeedController: holds context, debounces updates, reconciles sources
- ContextBridge: bridges context providers to controller
- ContextProvider: reactive + on-demand context value interface
- Branded ContextKey<T> for type-safe context keys

Moves source files to src/ directory and consolidates tests into
integration test.

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-core/src/index.ts

@@ -0,0 +1,23 @@
+// Context
+export type { Context, ContextKey } from "./context"
+export { contextKey, contextValue } from "./context"
+
+// Feed
+export type { FeedItem } from "./feed"
+
+// Data Source
+export type { DataSource } from "./data-source"
+
+// Context Provider
+export type { ContextProvider } from "./context-provider"
+
+// Context Bridge
+export { ContextBridge } from "./context-bridge"
+
+// Reconciler
+export type { ReconcileResult, ReconcilerConfig, SourceError } from "./reconciler"
+export { Reconciler } from "./reconciler"
+
+// Feed Controller
+export type { FeedControllerConfig, FeedSubscriber } from "./feed-controller"
+export { FeedController } from "./feed-controller"