web: reach-over-time chart with playhead, slice 5 of M3

Hand-rolled SVG as decided: dashed percent grid, one cumulative curve
per panel in its accent color, end labels via the shared formatPct, and
a playhead line with per-curve dots synced to the global round. The
cumulative-at-round counting moved into lib/reach.ts so the panel stats
and the chart cannot disagree.

web/src/lib/reach.ts

@@ -0,0 +1,5 @@
+// Cumulative reach at a playback round, shared by the panel stats and the
+// reach chart so both always agree with the network picture.
+export function reachedCountAtRound(reachedAtRound: number[], round: number): number {
+  return reachedAtRound.filter((reachedAt) => reachedAt >= 0 && reachedAt <= round).length
+}