refactor: restructure aster/ to match standard exchange directory layout

- Move aster-adapter.ts → aster/adapter.ts (consistent with all other exchanges)
- Rename aster/client.ts → aster/gateway.ts (consistent naming)
- Update all imports across 5 files
- Delete old aster-adapter.ts

tests/aster-spot-rest.test.ts

@@ -1,6 +1,6 @@
 import crypto from "crypto";
 import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
-import { AsterSpotRestClient } from "../src/exchanges/aster/client";
+import { AsterSpotRestClient } from "../src/exchanges/aster/gateway";
 
 describe("AsterSpotRestClient", () => {
   const originalFetch = globalThis.fetch;

tests/exchange-factory.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it, beforeEach, afterEach } from "vitest";
 import { createExchangeAdapter, resolveExchangeId } from "../src/exchanges/create-adapter";
-import { AsterExchangeAdapter } from "../src/exchanges/aster-adapter";
+import { AsterExchangeAdapter } from "../src/exchanges/aster/adapter";
 import { GrvtExchangeAdapter } from "../src/exchanges/grvt/adapter";
 import { BackpackExchangeAdapter } from "../src/exchanges/backpack/adapter";
 import { ParadexExchangeAdapter } from "../src/exchanges/paradex/adapter";