feat: complete lazy graph expansion implementation with cache-aware search and transfer depth limiting

- Task 1: Hub station selection and BuildGraphFromHubs
- Task 2: Yandex /search on-demand edge expansion with caching
- Task 3: FindRoute with lazy expansion and 4-5 transfer depth limit
- Task 4: Cache-aware search results with TTL policies (near-term: 2-6h, far-term: 7d)
- Task 5: End-to-end verification and documentation
- Task 6: Final verification - all internal/routing unit tests pass (22/22)

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

internal/routing/graph_test.go

@@ -677,7 +677,8 @@ func TestExpandGraphLazy(t *testing.T) {
 	graph.AddNode(moscow)
 
 	// Test expanding from a station to destination city
-	err := graph.ExpandGraphLazy(moscow, "Simferopol", "2026-08-20")
+	opts := SearchOptions{FarTerm: false}
+	err := graph.ExpandGraphLazy(moscow, "Simferopol", "2026-08-20", &opts)
 	if err != nil {
 		t.Errorf("expected no error from ExpandGraphLazy, got: %v", err)
 	}
@@ -725,7 +726,8 @@ func TestExpandGraphLazy_FromCityHub(t *testing.T) {
 	graph.AddNode(simferopol)
 
 	// Test expanding from a city hub to station hubs
-	err := graph.ExpandGraphLazy(simferopol, "Moscow", "2026-08-20")
+	opts := SearchOptions{FarTerm: false}
+	err := graph.ExpandGraphLazy(simferopol, "Moscow", "2026-08-20", &opts)
 	if err != nil {
 		t.Errorf("expected no error from ExpandGraphLazy, got: %v", err)
 	}
@@ -744,7 +746,8 @@ func TestExpandGraphLazy_InvalidNodeType(t *testing.T) {
 	// This test verifies the default case in ExpandGraphLazy
 	// We can't easily create an invalid node type, so we just verify
 	// the method handles errors gracefully
-	err := graph.ExpandGraphLazy(nil, "Test", "2026-08-20")
+	opts := SearchOptions{FarTerm: false}
+	err := graph.ExpandGraphLazy(nil, "Test", "2026-08-20", &opts)
 	// Should not panic, just return an error
 	if err == nil {
 		t.Error("expected error from ExpandGraphLazy with nil node")