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-post-processor.test.ts

@@ -1,6 +1,12 @@
 import { describe, expect, mock, test } from "bun:test"
 
-import type { ActionDefinition, FeedItem, FeedPostProcessor, FeedSource } from "./index"
+import type {
+	ActionDefinition,
+	ContextEntry,
+	FeedItem,
+	FeedPostProcessor,
+	FeedSource,
+} from "./index"
 
 import { FeedEngine } from "./feed-engine"
 import { UnknownActionError } from "./index"
@@ -471,7 +477,7 @@ describe("FeedPostProcessor", () => {
 		test("post-processors run during reactive context updates", async () => {
 			let callCount = 0
 
-			let triggerUpdate: ((update: Record<string, unknown>) => void) | null = null
+			let triggerUpdate: ((entries: readonly ContextEntry[]) => void) | null = null
 
 			const source: FeedSource = {
 				id: "aris.reactive",
@@ -502,7 +508,7 @@ describe("FeedPostProcessor", () => {
 			const countAfterStart = callCount
 
 			// Trigger a reactive context update
-			triggerUpdate!({ foo: "bar" })
+			triggerUpdate!([])
 			await new Promise((resolve) => setTimeout(resolve, 50))
 
 			expect(callCount).toBeGreaterThan(countAfterStart)
@@ -513,7 +519,7 @@ describe("FeedPostProcessor", () => {
 		test("post-processors run during reactive item updates", async () => {
 			let callCount = 0
 
-			let triggerItemsUpdate: (() => void) | null = null
+			let triggerItemsUpdate: ((items: FeedItem[]) => void) | null = null
 
 			const source: FeedSource = {
 				id: "aris.reactive",
@@ -543,7 +549,7 @@ describe("FeedPostProcessor", () => {
 			const countAfterStart = callCount
 
 			// Trigger a reactive items update
-			triggerItemsUpdate!()
+			triggerItemsUpdate!([weatherItem("w1", 25)])
 			await new Promise((resolve) => setTimeout(resolve, 50))
 
 			expect(callCount).toBeGreaterThan(countAfterStart)