exp: 轮次49——#31 门控地平线成立：终端段 20 mm 时时限收益与精度代价解耦

预注册判据(err_success_med 回到 ~0.22mm 且 time_ok>=0.31)由 20mm 臂满足：
0.2248 mm / 0.3281；到达题组内末态误差符号检验 67差/57好 p=0.42 ⇒ 与控制不可分
(pre h=2 是 82/42 p=4.2e-4，门控5mm 是 79/45 p=0.0029 仍显著变差)。
5→20mm 的分级响应是机理证据：病名是"终端代价的作用区间不够长"，不是"缺终端代价"。

不许只报好的那半：不存在同时占优的阈值——20mm 把 A 类到达打到 94/128(控制与5mm 都是 96)，
5mm 到达最高 0.5469 且行程比最好 2.4218 但精度仍显著变差。到达率在整条轴上都没跨过显著
(净 +3/+6/+5 题，McNemar p=0.68/0.33/0.42)，所以主口径仍写控制值。

四列 collision_rate/over_vmax_rate 全 0、n_plans_mean 全 195.0 ⇒ 没有放松判据、规划次数不变。
下一步 seed=1 复现(控制臂复用 round47_ep_s1)，判读规则已预注册。

docs/experiment_round49_gate20.json

@@ -0,0 +1,110 @@
+{
+  "config": {
+    "ckpt": "ml/runs/base_dag2_dag1/best.pt",
+    "samples": "ml/data/samples",
+    "tasks": null,
+    "split": "val",
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+    "K": 8,
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+    "kimax": 0.5,
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+    "sel_w": 0.1,
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+    "fb_cmin": 0.3,
+    "sub_refresh": 0,
+    "fb_block_gain": 0.5,
+    "dump": "ml/runs/round49_gate20_rows.json",
+    "out": "docs/experiment_round49_gate20.json",
+    "trace": null,
+    "select": "pre",
+    "replan": "empty",
+    "safety": 1,
+    "partial": 1,
+    "inc_cap": 1,
+    "gap_margin": 0.0,
+    "cert_batch": 1,
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+    "len_w": 0.0,
+    "tie": 0,
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+    "backoff": 0,
+    "max_sec": null,
+    "seed": 0,
+    "dls": "analytic",
+    "device": "cuda"
+  },
+  "batch_size": 256,
+  "batch_seed_rule": "same seed per batch",
+  "checkpoint_sha256": "f7f686fd8ee92fb6948fbcc4ee954636065ad87bb54f661c4f8f575bc16c2ce1",
+  "tasks_sha256": "249bd93457825cda98c59e64d9a096d3473c69aeb3553de1dea6d9e56be71772",
+  "kin_fp": "2ebcbf17",
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+    "brake_rate_p90": 0.35384616255760193,
+    "stuck_share": 0.2421875,
+    "n_plans_mean": 195.0,
+    "success_reachable": 0.5637860298156738,
+    "by_cat": {
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+      "D": [
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+        0
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+      "Dr": [
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+  },
+  "simulation_wall_s": 285.5599780000048
+}