Fix note bars hidden under alternating word-lane tint

.review-lane spans the full timeline box height (top:0; bottom:0) to
group each word with its note visually, but as a positioned element it
painted after the unpositioned .review-roll SVG, so the lane's
background covered the note bars for every other word. Give .review-roll
position: relative so DOM order (lanes, then notes) governs paint order
instead.

src/songtrace/static/index.html

@@ -330,7 +330,7 @@ button { cursor: pointer; }
 .review-lane-alt { background: var(--panel-raised); }
 .review-word-row { position: relative; height: 18px; }
 .review-word { position: absolute; top: 0; white-space: nowrap; font-size: 0.9rem; }
-.review-roll { display: block; width: 100%; height: 20px; margin-top: 4px; }
+.review-roll { position: relative; display: block; width: 100%; height: 20px; margin-top: 4px; }
 .review-note { fill: var(--aqua); }
 .review-note-low { fill: var(--warn); }
 .review-missing {