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-data-source-weatherkit/src/data-source.ts

@@ -1,6 +1,6 @@
-import type { Context, DataSource, FeedItemSignals } from "@aris/core"
+import type { Context, ContextKey, DataSource, FeedItemSignals } from "@aris/core"
 
-import { TimeRelevance } from "@aris/core"
+import { TimeRelevance, contextKey } from "@aris/core"
 
 import {
 	WeatherFeedItemType,
@@ -40,6 +40,13 @@ export interface WeatherKitQueryConfig {
 	units?: Units
 }
 
+interface LocationData {
+	lat: number
+	lng: number
+}
+
+const LocationKey: ContextKey<LocationData> = contextKey("aris.location", "location")
+
 export class WeatherKitDataSource implements DataSource<WeatherFeedItem, WeatherKitQueryConfig> {
 	private readonly DEFAULT_HOURLY_LIMIT = 12
 	private readonly DEFAULT_DAILY_LIMIT = 7
@@ -59,7 +66,8 @@ export class WeatherKitDataSource implements DataSource<WeatherFeedItem, Weather
 	}
 
 	async query(context: Context, config: WeatherKitQueryConfig = {}): Promise<WeatherFeedItem[]> {
-		if (!context.location) {
+		const location = context.get(LocationKey)
+		if (!location) {
 			return []
 		}
 
@@ -67,8 +75,8 @@ export class WeatherKitDataSource implements DataSource<WeatherFeedItem, Weather
 		const timestamp = context.time
 
 		const response = await this.client.fetch({
-			lat: context.location.lat,
-			lng: context.location.lng,
+			lat: location.lat,
+			lng: location.lng,
 		})
 
 		const items: WeatherFeedItem[] = []