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-source-location/src/location-source.ts

@@ -1,11 +1,11 @@
-import type { ActionDefinition, Context, FeedSource } from "@aris/core"
+import type { ActionDefinition, ContextEntry, FeedSource } from "@aris/core"
 
-import { UnknownActionError, contextKey, type ContextKey } from "@aris/core"
+import { Context, UnknownActionError, contextKey, type ContextKey } from "@aris/core"
 import { type } from "arktype"
 
 import { Location, type LocationSourceOptions } from "./types.ts"
 
-export const LocationKey: ContextKey<Location> = contextKey("location")
+export const LocationKey: ContextKey<Location> = contextKey("aris.location", "location")
 
 /**
  * A FeedSource that provides location context.
@@ -20,7 +20,7 @@ export class LocationSource implements FeedSource {
 
 	private readonly historySize: number
 	private locations: Location[] = []
-	private listeners = new Set<(update: Partial<Context>) => void>()
+	private listeners = new Set<(entries: readonly ContextEntry[]) => void>()
 
 	constructor(options: LocationSourceOptions = {}) {
 		this.historySize = options.historySize ?? 1
@@ -59,8 +59,9 @@ export class LocationSource implements FeedSource {
 		if (this.locations.length > this.historySize) {
 			this.locations.shift()
 		}
+		const entries: readonly ContextEntry[] = [[LocationKey, location]]
 		this.listeners.forEach((listener) => {
-			listener({ [LocationKey]: location })
+			listener(entries)
 		})
 	}
 
@@ -78,16 +79,16 @@ export class LocationSource implements FeedSource {
 		return this.locations
 	}
 
-	onContextUpdate(callback: (update: Partial<Context>) => void): () => void {
+	onContextUpdate(callback: (entries: readonly ContextEntry[]) => void): () => void {
 		this.listeners.add(callback)
 		return () => {
 			this.listeners.delete(callback)
 		}
 	}
 
-	async fetchContext(): Promise<Partial<Context> | null> {
+	async fetchContext(): Promise<readonly ContextEntry[] | null> {
 		if (this.lastLocation) {
-			return { [LocationKey]: this.lastLocation }
+			return [[LocationKey, this.lastLocation]]
 		}
 		return null
 	}