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-caldav/scripts/test-live.ts

@@ -5,6 +5,8 @@
  *   bun run test-live.ts
  */
 
+import { Context } from "@aris/core"
+
 import { CalDavSource } from "../src/index.ts"
 
 const serverUrl = prompt("CalDAV server URL:")
@@ -27,7 +29,7 @@ const source = new CalDavSource({
 	lookAheadDays,
 })
 
-const context = { time: new Date() }
+const context = new Context()
 
 console.log(`\nFetching from ${serverUrl} as ${username} (lookAheadDays=${lookAheadDays})...\n`)