Lessons learned:
Directed graphs need in/out degree checks: Unlike undirected where you count total degree, directed graphs require balancing in and out degrees.
Start and end are fixed: The start vertex has one extra outgoing edge, the end has one extra incoming edge.
Hierholzer works the same: The algorithm is identical; only the condition checking differs.
Don't forget connectivity: Even if degrees match, the graph must be connected.
Mail Delivery (undirected) and Teleporters Path (directed) form a complementary pair for learning Euler paths.