docs(tests): explain asset authorization and byte retention lifecycle

tests/headless/display-cache.test.mjs

@@ -36,6 +36,7 @@ test('last release aborts a load; a late source cannot replace or evict a newly
   const cancelled = assert.rejects(old.ready, { code: 'display_released' }); old.release(); await cancelled; assert.equal(signal.aborted, true);
   const next = cache.acquire(owner, 'asset', async () => ({ url: 'blob:new', release: () => newDisposed++ })); assert.equal(await next.ready, 'blob:new');
   gate.resolve({ url: 'blob:old', release: () => oldDisposed++ }); await tick(); assert.equal(oldDisposed, 1); assert.equal(newDisposed, 0);
+  // peer 的 load 一旦调用就抛错；成功读到 blob:new 证明迟到旧源没有逐出当前同键条目。
   const peer = cache.acquire(owner, 'asset', async () => { throw Error('unexpected duplicate read'); }); assert.equal(await peer.ready, 'blob:new'); peer.release(); next.release(); assert.equal(newDisposed, 1);
 });
 // 读取失败保留原错误码并移除失败条目，新 acquire 可重新读取成功，不永久缓存错误。
@@ -63,6 +64,7 @@ test('cache close rejects waiting readers immediately and disposes late data wit
   assert.throws(() => cache.acquire({}, 'asset', async () => ({ url: '/late' })), { code: 'display_scope_closed' }); cache.close();
 });
 // 空 URL 等非法源需清理已分配资源；某个 release 抛错通过 onError 报告，不能阻止其他 owner 的清理。
+// disposed 在这里直接累加每次底层 release 调用，与幂等句柄内部去重后的有效释放计数不同。
 test('invalid sources clean their allocation and cleanup errors do not interrupt other owners', async () => {
   const errors = [], cache = createDisplayCache({ onError: error => errors.push(error) }); let disposed = 0;
   const invalid = cache.acquire({}, 'invalid', async () => ({ url: '', release: () => disposed++ })); await assert.rejects(invalid.ready, { code: 'invalid_display_source' }); assert.equal(disposed, 1);