FLUX.3: match the released BFL reference image sizing #15068

Our FLUX.3 blocks only downsize a reference image when it exceeds 16 MP (Flux3ImageProcessor._resize_if_exceeds_area(image, 4096 * 4096) in modular_pipelines/flux3/encoders.py), then crop to the 16-pixel grid. The released BFL reference sizes references differently: a single reference is kept up to 4 MP and padded to the token grid (with crop-back on output), multiple references are scaled to 1 MP each, and the default output size follows the reference. We should match that in the blocks.

Posted by Claude Code on behalf of @yiyixuxu, with their approval.