feat: show playable charts with score hover and special-start markers

src/lib/models/chart-visual.ts

@@ -0,0 +1,15 @@
+export type ChartPosition = { timeSeconds: number; lane: number; width: number; control?: boolean };
+export type ChartVisualNote = ChartPosition & {
+  kind: 'tap' | 'flick' | 'hold' | 'relay';
+  direction?: 'up' | 'left' | 'right';
+  critical?: boolean;
+};
+export type ChartVisual = {
+  schemaVersion: 1;
+  songId: string;
+  difficulty: string;
+  durationSeconds: number;
+  laneCount: number;
+  notes: ChartVisualNote[];
+  holds: ChartPosition[][];
+};