For clique proofs, merge the pol lines

[ciaranm]

Rather than generating one pol line per colour class and then a pol line to add them together, generate a single pol line that does the whole step. Benchmark before and after, both with trimmed and untrimmed proofs.

See the end of "recovering at-most-one constraints" in https://www.dcs.gla.ac.uk/~ciaran/talks/2025-whoops.pdf for an example.