feat(PBI-49): add debugProps helper + Vitest test

Adds lib/debug.ts with debugProps(id, component, file) that returns
data-debug-id and data-debug-label attrs in dev mode, empty object in
production. Adds __tests__/lib/debug.test.ts covering both modes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/debug.ts

@@ -0,0 +1,16 @@
+export type DebugProps = {
+  'data-debug-id': string
+  'data-debug-label': string
+}
+
+export function debugProps(
+  id: string,
+  component: string,
+  file: string
+): DebugProps | Record<string, never> {
+  if (process.env.NODE_ENV === 'production') return {}
+  return {
+    'data-debug-id': id,
+    'data-debug-label': `${component} — ${file}`,
+  }
+}