feat(GraphTheory): embedding, chromatic number and minor

[alreadydone]

Add four eval tasks:

• Ringel–Youngs theorem (Heawood conjecture)
• Four color theorem
• Wagner's and Kuratowski's theorems on planar graphs
• Robertson–Seymour theorem

[kim-em]

🤖 Codex+Claude say: wagner_kuratowski is false as written: n5 and n33 are quantified but unused, so the last condition excludes only literal K₅ and K₃,₃ subgraphs. Subdividing one edge of K₅ gives a graph with a K₅ minor but neither literal forbidden subgraph. Could the tests use subdivide n5 and subdivide n33? That appears to match the intended statement.