test(scratch): leak-hunt probes for cava, render loop, decode stream

One-off diagnostics behind the FFTW leak fix: init/destroy x50 RSS
bound, Mach VM region walker, render-churn region growth, ffmpeg
decode-stream region/rss sampling.

tests/scratch-cava-reinit.test.ts

@@ -0,0 +1,23 @@
+/** Scratch: cava init/destroy cycles leak native fftw buffers? */
+import { test, expect } from "bun:test"
+import { loadCavaCore } from "../src/utils/cavacore"
+
+const cava = loadCavaCore()
+const skip = !cava
+
+test.skipIf(skip)("init/destroy x50: RSS bounded", () => {
+  const cfg = { bars: 64, sampleRate: 22050, channels: 1, autosens: 0 }
+  const samples = new Float64Array(8192)
+  Bun.gc(true)
+  const startRss = process.memoryUsage.rss()
+  for (let i = 0; i < 50; i++) {
+    cava!.init(cfg)
+    cava!.execute(samples)
+    cava!.destroy()
+  }
+  Bun.gc(true)
+  const endRss = process.memoryUsage.rss()
+  const grown = (endRss - startRss) / 1048576
+  console.log(`init/destroy x50: rss delta=${grown.toFixed(1)}MB`)
+  expect(grown).toBeLessThan(100)
+}, 60_000)