E1 halting-gate machinery (adapter, trainer, gated eval) + pre-registration item 18

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

results-loop/PROTOCOL_UNIFIED.md

@@ -359,3 +359,21 @@ on GSM alone in the correct regime, delivers no usable gain and the
 standard fidelity damage; combined with the scope note, the boundary
 claim (structural: supervision density + state-evolution bottleneck)
 is fully supported.
+
+18. **E1: learned per-prompt halting gate (pre-registered 2026-07-16
+    ~00:20, before any arm runs; PLAN_SELFPACED.md).** HaltingMergeAdapter:
+    frozen-recipe merge + ACT-style halting head on the last prompt
+    position's workspace state; soft state-mixture training, CE + lambda *
+    E[iters], penalty warmup at step 100; NO difficulty curriculum (mixed
+    batches — the gate must discover the allocation). k_max=4, e400/e600
+    checkpoints, deploy = sequential halting at 0.5 cumulative mass,
+    generation via frozen-prompt at per-item k*. Arms: lambda in
+    {0, 1e-3, 1e-2}, seed 0. Eval: 250 items, vs anchors k=0 (0.488),
+    uniform merge k=4 (0.512/0.885/0.464), probe-gate E0 (0.520/0.975/0.286).
+    Predictions: (a) some lambda gives overall >= 0.512 at mean E[k] <=
+    2.4 (60% of uniform-4); (b) easy >= 0.95 at that lambda; (c) k*-vs-hard
+    point-biserial r > 0.3; (d) hard >= 0.286 (beats E0's frozen probe).
+    Collapse (E[k] pinned at 1 or 4 for all lambda) falsifies E1 and
+    triggers the plan's kill criterion. lambda=0 control isolates whether
+    the CE gradient alone moves the gate (expected: barely — penalty
+    provides the pressure).