Constant MAX_FAN_WIDTH 

pub const MAX_FAN_WIDTH: u32 = 4096;

The largest fan width, tune or test, a study configuration may declare.

Two limits meet here and the tighter one wins. The correctness limit is TEST_REPLICATE_OFFSET: at or above it the tune stream would reach into the test stream’s replicate indices and a configuration could be tested on a seed it was tuned on, while test_seed’s saturating add would collapse every high replicate onto one stream. The practical limit is far lower — every replicate in a fan is a whole evaluation of the objective, held in memory together — so the enforced cap sits where a fan is still a fan rather than an accident, and a study that wants more is asking for something the aggregate was never meant to average.