Applies the MultiVectorEncoder integration from PR #1 by @tomaarsen, verified against the ColPali path (identical shapes, MaxSim within bfloat16 noise).
Also ships bidirectional.py: released colpali-engine (through 0.3.17) exposes no enable_bidirectional_attention, so infer.py and reproduce.py raised AttributeError on a plain install, and running causal shifts top-hit MaxSim by about 1.1.
