test(mediabunny): [PKG-MB-02] establish native input and lifecycle baselines

test/helpers/trackless-mp4.js

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+import assert from 'node:assert/strict'
+import { Buffer } from 'node:buffer'
+
+// Derive a trackless container from the small, repository-owned MP4 fixture.
+export function tracklessMp4(bytes) {
+  function boxes(bytes) {
+    const result = []
+    let offset = 0
+    while (offset < bytes.length) {
+      assert(offset + 8 <= bytes.length)
+      const size = bytes.readUInt32BE(offset)
+      assert(size >= 8 && offset + size <= bytes.length, 'Fixture requires bounded 32-bit boxes')
+      result.push({ type: bytes.toString('ascii', offset + 4, offset + 8), bytes: bytes.subarray(offset, offset + size) })
+      offset += size
+    }
+    return result
+  }
+  const top = boxes(bytes)
+  const ftyp = top.find(box => box.type === 'ftyp')
+  const moov = top.find(box => box.type === 'moov')
+  assert(ftyp && moov)
+  const children = boxes(moov.bytes.subarray(8))
+  assert(children.some(box => box.type === 'trak'))
+  const body = Buffer.concat(children.filter(box => box.type !== 'trak').map(box => box.bytes))
+  const header = Buffer.alloc(8)
+  header.writeUInt32BE(body.length + 8)
+  header.write('moov', 4)
+  return Buffer.concat([ftyp.bytes, header, body])
+}