VAST-2.0-linear-tracking-quartiles

<Linear> has no standard quartile tracking events — measurement system receives no signal

A `<Linear>` creative with no `<TrackingEvents>` containing any of `start`, `firstQuartile`, `midpoint`, `thirdQuartile`, or `complete` will serve and deliver impressions but return zero measurement signal. The advertiser's reporting system has no way to verify viewability or completion rates. While the VAST spec marks `<TrackingEvents>` as optional (minOccurs=0), omitting all five standard quartile events is a complete measurement blackout in practice.

<Linear>
  <Duration>00:00:30</Duration>
  <!-- no <TrackingEvents> — measurement system receives no signal -->
  <MediaFiles>
    <MediaFile delivery="progressive" type="video/mp4" width="1280" height="720">
      <![CDATA[https://example.com/ad.mp4]]>
    </MediaFile>
  </MediaFiles>
</Linear>

<Linear>
  <Duration>00:00:30</Duration>
  <TrackingEvents>
    <Tracking event="start"><![CDATA[https://example.com/track?e=start]]></Tracking>
    <Tracking event="firstQuartile"><![CDATA[https://example.com/track?e=q1]]></Tracking>
    <Tracking event="midpoint"><![CDATA[https://example.com/track?e=mid]]></Tracking>
    <Tracking event="thirdQuartile"><![CDATA[https://example.com/track?e=q3]]></Tracking>
    <Tracking event="complete"><![CDATA[https://example.com/track?e=complete]]></Tracking>
  </TrackingEvents>
</Linear>