feat(frontend): annotate key numbers with meaning (#18)

Add shared Meaning primitive and pure interpretation helpers for interval,
tyre age, and championship gap columns on the live tower, tyre deg panel,
and championship hub. Thresholds are exported consts (undercut window tied
to PIT_LOSS_SECONDS from #13).

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

frontend/src/test/Meaning.test.tsx

@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import { Meaning } from '../components/Meaning'
+
+describe('Meaning', () => {
+  it('renders bare value when meaning is null', () => {
+    render(<Meaning value="+1.234" meaning={null} />)
+    expect(screen.getByText('+1.234')).toBeInTheDocument()
+    expect(screen.queryByText('DRS range')).not.toBeInTheDocument()
+  })
+
+  it('renders value with caption and tooltip title', () => {
+    render(
+      <Meaning
+        value="+0.4"
+        meaning="DRS range"
+        title="Within 1.0s — DRS enabled next straight"
+        tone="good"
+      />,
+    )
+    const value = screen.getByText('+0.4')
+    expect(value).toBeInTheDocument()
+    expect(screen.getByText('DRS range')).toHaveClass('meaning-caption--good')
+    expect(value.closest('.meaning')).toHaveAttribute('title', 'Within 1.0s — DRS enabled next straight')
+  })
+})