capsite/lib: MapStore and patterns work-alikes

Enough of @agoric/store's MapStore and @endo/patterns' M for the
credential-rotation design: makeMapStore(label, { keyShape, valueShape },
kv) keeps entries in a DO's sync KV under store:<label>:, and readOnly()
hands out the queries alone. Facets are workerd RpcTargets in place of
endo's Far.

Unlike endo, error messages never include keys or values: the values
here are credentials, and messages cross RPC into logs.

Generated-by: Claude Opus 5.5

lib/tests/test-worker.ts

@@ -0,0 +1,15 @@
+/** @file the tests' main module: a DO that hosts stores on its own storage. */
+import { DurableObject } from "cloudflare:workers";
+import { M } from "../src/patterns.js";
+import { makeMapStore, type MapStore } from "../src/store.js";
+
+/** hosts the store from the credential-rotation sketch: one key, `cookie` */
+export class StoreHost extends DurableObject {
+	cookies(): MapStore<string> {
+		return makeMapStore<string>("cookie", { keyShape: "cookie", valueShape: M.string() }, this.ctx.storage.kv);
+	}
+}
+
+export default {
+	fetch: () => new Response("store tests", { status: 404 }),
+};