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/helpers.ts

@@ -36,6 +36,14 @@ export async function gotoRaceHubReady(page: Page, sessionKey = FULL_SESSION): P
   await waitForScreenshotReady(page)
 }
 
+export async function gotoRaceStoryReady(page: Page, sessionKey = FULL_SESSION): Promise<void> {
+  await page.goto(`/race-hub?session_key=${sessionKey}`)
+  await expect(page.getByTestId('race-hub')).toBeVisible()
+  await page.getByRole('tab', { name: 'Race Story' }).click()
+  await expect(page.getByTestId('position-chart')).toBeVisible()
+  await waitForScreenshotReady(page)
+}
+
 export async function gotoDataLibraryReady(page: Page): Promise<void> {
   await page.goto('/admin')
   await expect(page.getByTestId('data-library')).toBeVisible()