exp: 轮次48——打分地平线四点曲线(h=2/4/8/16)：进度单调、精度代价阶跃在 h<=4、h=8 是坏点

时限内 0.2656/0.2813/0.3047/0.3320；2 s 误差比逐题符号检验随 h 缩短单调增强
(h=8 166:90 p=2.4e-6、h=4 184:72 p=1.8e-12、h=2 192:64 p=4.9e-16)；
到达四点全在噪声内(净 -1/+4/+5，McNemar p>=0.42)。
末态到达误差 0.2175/0.2255/0.3301/0.4151 mm ⇒ 代价集中在 h<=4，病因是"近终段没有终端代价"
而不是"前瞻短"，故派生 #31(门控地平线)、按预注册规则作废 #26。
h=8 行程比 2.6749 反而比控制 2.6158 更差 ⇒ "地平线砍一半"的渐进方案被中间点自己否掉。

判据与成本(实测非推算)：四点 collision_rate=0、over_vmax_rate=0、start_fail=0，
speed_ratio/accel_ratio 四点逐位相同，n_plans_mean 全为 195.0 ⇒ 规划次数与认证调用数不变。

docs/experiment_round48_pre_h4.json

@@ -0,0 +1,109 @@
+{
+  "config": {
+    "ckpt": "ml/runs/base_dag2_dag1/best.pt",
+    "samples": "ml/data/samples",
+    "tasks": null,
+    "split": "val",
+    "n": 256,
+    "K": 8,
+    "commit": 2,
+    "fb": 2.2,
+    "ki": 0.0,
+    "fb_scale": 0.0,
+    "fb_near": 0.2,
+    "fb_fallback": 0.0,
+    "kdecay": 0.9,
+    "kimax": 0.5,
+    "fb_every": 1,
+    "safety_try": 0,
+    "sel_w": 0.1,
+    "dls_safety": 1,
+    "fb_clear": 0.0,
+    "fb_cmin": 0.3,
+    "sub_refresh": 0,
+    "fb_block_gain": 0.5,
+    "dump": "ml/runs/round48_pre_h4_rows.json",
+    "out": "docs/experiment_round48_pre_h4.json",
+    "trace": null,
+    "select": "pre",
+    "replan": "empty",
+    "safety": 1,
+    "partial": 1,
+    "inc_cap": 1,
+    "gap_margin": 0.0,
+    "cert_batch": 1,
+    "steps": 1,
+    "z_scale": 1.0,
+    "len_w": 0.0,
+    "tie": 0,
+    "pre_h": 4,
+    "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",
+  "act_scale": 1.0,
+  "metrics": {
+    "n": 256,
+    "n_ref": 256,
+    "success": 0.5390625,
+    "collision_free_success": 0.5390625,
+    "time_ok_1p3": 0.3046875,
+    "collision_rate": 0.0,
+    "collision_steps_mean": 0.0,
+    "travel_ratio_median": 2.5618672370910645,
+    "time_mean_s": 2.4820291996002197,
+    "brakes_mean": 20.40625,
+    "start_fail": 0.0,
+    "goal_ratio_2s": 0.23497404158115387,
+    "err_fail_med": 199.24868774414062,
+    "err_success_med": 0.33005011081695557,
+    "dls_share": 0.62890625,
+    "time_budget": "teacher×2(≥3 s)",
+    "speed_ratio_p50": 1.0000016689300537,
+    "speed_ratio_p95": 1.0000101327896118,
+    "speed_ratio_max": 1.0000101327896118,
+    "over_vmax_rate": 0.0,
+    "accel_ratio_p95": 1.0000016689300537,
+    "accel_ratio_max": 1.0000101327896118,
+    "reverse_p50": 0.24242424964904785,
+    "reverse_mean": 0.24662044644355774,
+    "reverse_moving_p50": 0.30327868461608887,
+    "moving_share_p50": 0.9583333134651184,
+    "brake_rate_mean": 0.1046474277973175,
+    "brake_rate_p50": 0.0051282052882015705,
+    "brake_rate_p90": 0.3589743673801422,
+    "stuck_share": 0.24609375,
+    "n_plans_mean": 195.0,
+    "success_reachable": 0.5679012537002563,
+    "by_cat": {
+      "A": [
+        128,
+        96
+      ],
+      "B": [
+        74,
+        22
+      ],
+      "C": [
+        32,
+        13
+      ],
+      "D": [
+        13,
+        0
+      ],
+      "Dr": [
+        9,
+        7
+      ]
+    }
+  },
+  "simulation_wall_s": 349.25640099999146
+}