docs: phase 73 postscript - alpha dose-response confirms 1.4 optimum; no second A/B

EXPERIMENTS.md

@@ -3668,3 +3668,9 @@ private. SELECTION: 62e9d91 (public 1480.85) PRIMARY; 4c93883
 to decide whether ONE more A/B is justified - stopping rule: only if
 the CV curve is still falling at 1.6+; iterating alpha values on
 public beyond one more point = the probing pattern we reject.
+
+Phase 73 postscript - dose-response complete: alpha 1.4 -> 1498.3
+(-43.0), 1.6 -> 1562.1 (+20.8), 1.8 -> 1573.7 (+32.4), 2.0 -> 1589.0
+(+47.7). Clear minimum at 1.4; monotone rise beyond. NO second public
+A/B per the stopping rule. FINAL: select 62e9d91 (1480.85); fallback
+4c93883 (1575.31).