schedopt-v3-mechanism

Phase E of the v3 campaign. The descriptive result — a high-start, late-diving concentration schedule truncates the tail of stalling seeds — leaves the mechanism open. This node measures, per seed and along training, when the agreement rule is built and when it erodes; whether the late dive plays the role a learning-rate cooldown plays in the warmup-stable-decay picture; and whether stalled seeds resemble delayed, grokking-like generalization. Ablations that remove the high start, the hold, or the late dive — and mid-run branch interventions that inject a concentration excursion into stalled fixed-$\alpha$ runs — convert correlation into cause by showing which component reintroduces the tail and which intervention rescues it.