refactor(studio): name the non-keyframe header for what it is

LegacyTrackHeader reads as deprecated code. It is the live path for every
track that has no keyframe clip to disclose, so call it PlainTrackHeader and
say so in a comment.

packages/studio/src/player/components/TimelineTrackHeader.tsx

@@ -75,7 +75,9 @@ function VisibilityButton({
   );
 }
 
-function LegacyTrackHeader({
+// The header a track gets when it has no keyframe clip to disclose: label, clip
+// count, eye. Not deprecated — it is the live path for every track without lanes.
+function PlainTrackHeader({
   trackNumber,
   trackLabel,
   clipCount,
@@ -318,7 +320,7 @@ export function TimelineTrackHeader({
       }}
     >
       {!keyframeClip || lanes.length === 0 ? (
-        <LegacyTrackHeader
+        <PlainTrackHeader
           trackNumber={trackNumber}
           trackLabel={trackLabel}
           clipCount={clipCount}