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/context.ts

@@ -0,0 +1,46 @@
+/**
+ * Branded type for type-safe context keys.
+ *
+ * Each package defines its own keys with associated value types:
+ * ```ts
+ * const LocationKey: ContextKey<Location> = contextKey("location")
+ * ```
+ */
+export type ContextKey<T> = string & { __contextValue?: T }
+
+/**
+ * Creates a typed context key.
+ *
+ * @example
+ * ```ts
+ * interface Location { lat: number; lng: number; accuracy: number }
+ * const LocationKey: ContextKey<Location> = contextKey("location")
+ * ```
+ */
+export function contextKey<T>(key: string): ContextKey<T> {
+	return key as ContextKey<T>
+}
+
+/**
+ * Type-safe accessor for context values.
+ *
+ * @example
+ * ```ts
+ * const location = contextValue(context, LocationKey)
+ * if (location) {
+ *   console.log(location.lat, location.lng)
+ * }
+ * ```
+ */
+export function contextValue<T>(context: Context, key: ContextKey<T>): T | undefined {
+	return context[key] as T | undefined
+}
+
+/**
+ * Arbitrary key-value bag representing the current state.
+ * Always includes `time`. Other keys are added by context providers.
+ */
+export interface Context {
+	time: Date
+	[key: string]: unknown
+}