feat: Puente Mayúscula-Minúscula + TTS SpeechSynthesis + E2E fixes

app/api/curriculum/next/route.ts

@@ -0,0 +1,13 @@
+import { NextRequest, NextResponse } from "next/server"
+import { getNextExercise } from "@/lib/curriculum/engine"
+
+export async function GET(request: NextRequest) {
+  const childId = request.nextUrl.searchParams.get("childId")
+  const topic = (request.nextUrl.searchParams.get("topic") || "lectura") as "lectura" | "numeros"
+  if (!childId) {
+    return NextResponse.json({ error: "childId required" }, { status: 400 })
+  }
+
+  const exercise = await getNextExercise(childId, topic)
+  return NextResponse.json(exercise || { id: null, done: true })
+}