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-stream-leak.test.ts

@@ -0,0 +1,36 @@
+/** Scratch: does the ffmpeg decode stream leak VM regions per chunk? */
+import { test, expect } from "bun:test"
+import { EpisodePcmCache } from "../src/utils/audio-pcm-cache"
+import { spawnSync } from "child_process"
+
+const wav = "/tmp/podtui-stream.wav"
+if (!(await Bun.file(wav).exists()) && (await Bun.$`which ffmpeg`.nothrow())) {
+  await Bun.$`ffmpeg -f lavfi -i "sine=frequency=440:duration=600" -ar 22050 -ac 1 -sample_fmt s16 ${wav}`.quiet().nothrow()
+}
+
+function regions(): number {
+  const out = spawnSync("vmmap", [String(process.pid)], { timeout: 20000 }).stdout?.toString() ?? ""
+  return out.split("\n").filter((l) => l.includes("VM_ALLOCATE")).length
+}
+function rss(): number {
+  return Number(spawnSync("ps", ["-o", "rss=", "-p", String(process.pid)]).stdout?.toString().trim() || 0)
+}
+
+test("decode stream 90s: regions and rss bounded", async () => {
+  Bun.gc(true)
+  await Bun.sleep(200)
+  const r0 = regions(), m0 = rss()
+  const pcm = new EpisodePcmCache({ url: wav })
+  pcm.startDecode(0)
+  const t0 = Date.now()
+  while (Date.now() - t0 < 90_000) {
+    await Bun.sleep(5_000)
+    const pos = ((Date.now() - t0) / 1000) * 4
+    pcm.readWindow(new Float64Array(512), pos)
+  }
+  Bun.gc(true)
+  const r1 = regions(), m1 = rss()
+  console.log(`stream 90s: regions ${r0}->${r1} (delta ${r1 - r0}), rss ${(m0 / 1048576) | 0}->${(m1 / 1048576) | 0}MB`)
+  pcm.stop()
+  expect(r1 - r0).toBeLessThan(100)
+}, 140_000)