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-region-count.ts

@@ -0,0 +1,34 @@
+/** Mach VM region walker via FFI (self-process). */
+import { dlopen, FFIType, ptr } from "bun:ffi"
+
+const k = dlopen("/usr/lib/system/libsystem_kernel.dylib", {
+  mach_task_self: { args: [], returns: FFIType.u64 },
+  mach_vm_region: {
+    args: [FFIType.u64, FFIType.ptr, FFIType.ptr, FFIType.u32, FFIType.ptr, FFIType.ptr, FFIType.ptr],
+    returns: FFIType.i32,
+  },
+})
+
+/** Walk own VM map; count total regions and ~128K ones. */
+export function countRegions(): { total: number; r128k: number } {
+  const task = (k.symbols.mach_task_self as any)() as number
+  const addr = new BigUint64Array(1)
+  const size = new BigUint64Array(1)
+  const info = new Uint32Array(16)
+  const infoCnt = new Uint32Array(1)
+  const objectName = new Uint32Array(1)
+  let total = 0
+  let r128k = 0
+  addr[0] = 1n
+  const walk = k.symbols.mach_vm_region as any
+  for (;;) {
+    infoCnt[0] = 16
+    const kr = walk(BigInt(task), ptr(addr), ptr(size), 9, ptr(info), ptr(infoCnt), ptr(objectName))
+    if (kr !== 0) break
+    total++
+    if (size[0] >= 131072n && size[0] <= 139264n) r128k++
+    addr[0] = addr[0] + size[0]
+    if (addr[0] === 0n || total > 500000) break
+  }
+  return { total, r128k }
+}