test: add integration test suite — 74 tests passing against local TS3 server

Fix @discordjs/opus ESM import for Node 24 compatibility.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/audio/encoder.ts

@@ -1,4 +1,5 @@
-import { OpusEncoder } from "@discordjs/opus";
+import opusModule from "@discordjs/opus";
+const { OpusEncoder } = opusModule;
 
 const SAMPLE_RATE = 48000;
 const CHANNELS = 2;