feat: replace flat context with tuple-keyed store (#50)

Context keys are now tuples instead of strings, inspired by
React Query's query keys. This prevents context collisions
when multiple instances of the same source type are registered.

Sources write to structured keys like
["aris.google-calendar", "nextEvent", { account: "work" }]
and consumers can query by prefix via context.find().

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

packages/aris-core/src/feed-source.ts

@@ -1,5 +1,5 @@
 import type { ActionDefinition } from "./action"
-import type { Context } from "./context"
+import type { Context, ContextEntry } from "./context"
 import type { FeedItem } from "./feed"
 
 /**
@@ -57,7 +57,7 @@ export interface FeedSource<TItem extends FeedItem = FeedItem> {
 	 * Maps to: source/contextUpdated (notification, source → host)
 	 */
 	onContextUpdate?(
-		callback: (update: Partial<Context>) => void,
+		callback: (entries: readonly ContextEntry[]) => void,
 		getContext: () => Context,
 	): () => void
 
@@ -67,7 +67,7 @@ export interface FeedSource<TItem extends FeedItem = FeedItem> {
 	 * Return null if this source cannot provide context.
 	 * Maps to: source/fetchContext
 	 */
-	fetchContext(context: Context): Promise<Partial<Context> | null>
+	fetchContext(context: Context): Promise<readonly ContextEntry[] | null>
 
 	/**
 	 * Subscribe to reactive feed item updates.