ResultLinear leaf refit to the depth-4 root value: held-out fit, depth-5 agreement proxy, and gameplay pilot

Stage 1: on the EX-4 panel's fit games (2,913 roots with depth-4 root value above -50,000; 425 dropped), ridge regression of the depth-4 root value on the eighteen fair-leaf terms plus intercept (lambda 0.001 by game-grouped CV, the grid edge) reaches held-out R^2 0.852 on 1,877 held-out roots, against 0.729 for an affine rescaling of the frozen leaf (gain +0.123; P-L1 passes) and 0.852 with the best-immediate-six-feature-drop term added (gain -0.0001; P-L3 fails as predicted: the six drop features carry no leaf-relevant information the fair terms lack). The refit reweights the terms wildly relative to the frozen constants (open columns -4,232 against +180, solid exposure +3,273 against +40, intercept +45,744), which correlated terms permit. Stage 2, the disclosed biased proxy: full-width d4s7 with the refit leaf agrees with exact frozen-leaf d5s7 on 68.0% of the 128 sub-panel roots against 79.7% for the frozen leaf and loses 9,350 points per decision under those values against 146 (paired change -9,204, bound -21,308); it changes 28 of 128 decisions. Stage 3: in play on the 256 pilot games, full-width fair d4s7 with the refit leaf averages 321,045 points and 93.74 moves against 404,497 and 116.41 for the frozen leaf on the same seeds: paired delta -83,451 points (one-sided 95% bounds -118,022 and -49,021; floor 34,345) and -22.66 moves (bounds -31.98, -13.32), 104 wins to 152; upper bound below zero, so P-L2 FAILS with a measurable loss of about a fifth of the score. The leaf that predicts the depth-4 value best is a worse leaf: it explains more variance among live roots by absorbing four plies of expected score, and inside the search that reweighting mis-prices the positions the search actually has to choose between, above all near death, which the fit deliberately excluded. Unlike three earlier proxies, this one pointed the right way. Valid negative result at pilot tier; one-step linear TreeStrap on these eighteen terms is closed as tested.