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The experiment should isolate the consequence of moving occupancy through a permanent random perfect matching immediately before normal Life. Periodic mode tests a synchronized field-wide exchange cadence. Neighbor-trigger mode makes the same exchange conditional on local crowding and lets one member's local environment alter both members' inputs to Life.

Success is not merely a correct random permutation of states. A visitor must be able to see when a pair was commanded, whether unlike states actually crossed, which endpoint caused a neighbor-triggered exchange, and how the swapped intermediate state then survived, died, or gave birth under Life. Long-run tests must record whether the authored regimes sustain activity and produce abrupt changes rather than assuming that they will.

Rules, update order and replay

The world is an even-sized square binary lattice. A seeded Fisher-Yates shuffle divided into adjacent shuffled entries creates one symmetric, self-excluding perfect matching. Every position has exactly one counterpart, every pair is disjoint, and the matching persists until Reset, field-size change, initial- condition reconstruction, or a new world seed.

Each Step has two synchronous stages:

1. The exchange stage reads one frozen current board. In Periodic mode every pair is commanded on Steps N, 2N, 3N, and so on, where N is the configured interval. In Neighbor count mode each endpoint independently compares its frozen Moore-neighbor count with the configured exact-count mask. If either endpoint matches, the pair is commanded.
2. Every commanded pair exchanges its two occupancy values exactly once. If both endpoints match, that is one exchange, not two cancelling exchanges. If both occupancies are equal, the command is retained as causal evidence even though it produces no effective state movement.
3. Ordinary configurable Life then reads the complete exchanged intermediate board and commits all final occupancies simultaneously. Life never reads a mixture of pre-exchange and post-exchange values.

The local Moore counts used to trigger an exchange come from the original frozen board. The counts used by Life come from the exchanged intermediate board. Both respect the current toroidal or bounded-edge view. A selected cell's trigger is reported as Selected, Counterpart, Both, Periodic, or None; there is no priority between endpoints and no iteration-order tie-breaker.

Step performs this complete exchange-then-Life transition once. Reset rebuilds the selected occupancy field, seeded perfect matching, generation zero, and empty event evidence while preserving the authored transition configuration. Manual painting preserves the matching and generation but clears old event evidence. Exact replay requires configuration, selected initial condition or occupancy, density when random, topology, world seed, generation, partner indices, board occupancy, and all retained last-Step evidence. Evolution makes no random calls.

Reading the simulation

The diagnostic renderer uses lime for established living cells, cream for Life births from a dead intermediate state, and gold for an alive state received from the counterpart that survives Life. Simplified Color mode reduces occupancy to dead/alive without removing written evidence.

Pair inspection pauses painting, outlines the selected cell's current Moore neighbors, draws one line to its only counterpart, and outlines that counterpart separately. Written evidence reports both coordinates and occupancies; both frozen trigger counts; whether Selected, Counterpart, Both, or Periodic commanded the pair; whether unlike occupancies would effectively cross; selected occupancy before and after exchange; its Life neighbor count on the fully exchanged board; the ordinary Life result; last-Step command/effective-swap evidence; and the exact next result. The canvas label states the same sequence so color and geometry are not the only explanation.