feat(race-story): polish chapter strip, controls, graph, and map UX (#68)

Consolidate the Race Story section into a cohesive timeline surface: custom
chapter-strip scrolling with active-card sync, unified segmented playback
controls, chapter bands on the position graph with decimated axes and
de-collided labels, map toggle gated by an on-mount replay/outline probe,
and shared empty-state cards. Adds vitest coverage and visual snapshots.

Note: replay/outline queries now probe on mount (not only when Map opens) so
the toggle can be hidden before users hit a dead panel.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/visual/race-story.spec.ts

@@ -0,0 +1,9 @@
+import { test } from '@playwright/test'
+import { gotoRaceStoryReady, screenshotPage } from './helpers'
+
+test.describe('Race Story visual regression', () => {
+  test('race-story', async ({ page }) => {
+    await gotoRaceStoryReady(page)
+    await screenshotPage(page, 'race-story')
+  })
+})