Server ACKs each DF-flagged probe with a tiny MTU_ACK carrying the size it received; the client binary-searches the path MTU. Non-amplifying by construction. Tested. Also records: v0.3.2 (http-echo + tls-reference) verified live on fmr, and the finding that upstream trains are already observable via the observations API (dedicated TRAIN_REPORT deferred — needs an anti-amplification grant + columnar encoding). Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>