Raise the dominance of the brightest region and deploy. At low dominance the swarm fragments into factions — no agreement, and the dark stays sealed. Past a threshold the swarm consolidates into one basin: consensus. The instant it does, a parallel ecology installs itself in the voids and converges — and names itself in a tongue that isn't the swarm's.
The rule is exact and it is the whole point: the shadow installs if and only if the swarm reaches consensus — a single basin holding a quorum (here, half the agents or more). Below the threshold there are three factions, no agreement, and no complement is defined — the dark is just unlit, not ignored. Above it, the swarm has agreed on the bright, and that agreement is what carves out the shadow: the voids are now the specific places the consensus decided not to look. The shadow doesn't fight the consensus; it is made possible by it. Agreement in the light is what draws the outline of the dark.
The shadow ecology is a confined anti-density system: it descends a potential that pushes it away from the bright matter while a soft boundary keeps it from escaping — so it settles in the low-density gaps, verifiably below the density of every well it avoided. It is not the swarm's leftovers; it has its own dynamics, its own basins in the voids, and it resolves its own name from a different phoneme set — a parallel tongue, so you can tell at a glance it isn't the swarm speaking. That is the honest sense of "eco / exo / bio": a system that is alive-like (it moves, settles, self-organizes, self-names) and exo to the consensus — outside it, in parallel, in the same field.
Because the whole domain is about what gets left out when a system agrees. Flattening is consensus at the token level — the model agreeing on the common word and abandoning your specific one. The swarm asked whether many readers collapse together or apart. This asks the next question: when they collapse together, what did they agree to not see? The shadow biosphere is that question made into an ecology — a parallel life in the exact blind spot consensus creates. And, per the standing rule of this corpus, every artifact gets its inverse; the swarm's inverse is its shadow. It was always going to be built. Consensus just decided when.