exp: #21定案——关闸后两段每格都不更差(碰撞步58.3→45.1)，主因是候选塌缩35倍；安全层值25.4pp到达但净收益为正(纯策略无碰撞到达仅0.359)

2026-10-20 22:53:07 +08:00
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@@ -107,6 +107,24 @@
 - **对 `backoff` 的预期与判据**：全拒意味着"连第 1 步的端点或中点已经穿进障碍"，缩步能不能救取决于**穿得有多深**（穿 0.1 mm 救得回，穿 3 mm 救不回）。这个数转储里没记（只记被选中那条），所以直接由 `backoff_mean`/`reject_all_mean` 来量：
  `reject_all_mean ≈ 原全拒次数`（=backoff_mean+brakes_mean，与轮次 33 cap 的 19.55 可比），`backoff_mean/(backoff_mean+brakes_mean)` 就是"缩步能救回的比例"。这是补跑（#22）要读的第一个数。

+## 判别定案（#21）：两段动作不是"更危险"，是"没有备选"；顺带量出安全层值多少到达
+
+- 关安全层（`--safety 0`，同 256 题、`--inc_cap 1`，只改段数）：
+
+  | 口径 | 到达 | **无碰撞到达** | 碰撞率 | 碰撞步/题 | 时限内 | 行程比 |
+  |---|---|---|---|---|---|---|
+  | pure steps=1 | 0.7773 | 0.3594 | 0.5859 | 58.34 | 0.5352 | 2.375 |
+  | pure steps=2 | **0.8164** | **0.3750** | **0.5586** | **45.07** | **0.6836** | **1.594** |
+  | 开安全层 steps=1（对照） | 0.5234 | 0.5234 | 0 | 0 | 0.2656 | 2.616 |
+  | 开安全层 steps=2 | 0.3945 | 0.3945 | 0 | 0 | 0.2969 | 1.844 |
+
+  （`docs/experiment_round40_safetyoff_steps{1,2}.json`）
+- **结论 1（判据 #21 定案）**：关掉安全层以后，两段在**每一格都不更差**——到达更高（+3.9 pp）、时限内高一大截（+14.8 pp）、**碰撞步反而更少**（58.3→45.1）、行程比更接近老师。⇒ "两段动作更危险"被否；两段在开环与纯策略口径都更好，它唯一输的地方是"开安全层时刹车多 4.5 倍"。
+- **结论 2（机理）**：这正好是候选塌缩的必然结果——`steps=2` 时 K 条候选的一阶步标准差从 0.852 掉到 0.0245（35 倍），"塌成一条"以后，那条在采样点上穿模就等于**全拒**；`steps=1` 时候选彼此差得远，总有一条能过。所以"没有备选"是主因，不是"界太保守"（线上只有端点+中点点检，没有位移界）。
+- **结论 3（安全层值多少到达，首次定量）**：一步口径下安全层把到达从 0.7773 压到 0.5234（**−25.4 pp**），换来碰撞从 58.6% 到 0；纯策略的**无碰撞到达只有 0.3594**，比开安全层的 0.5234 低 16.4 pp ⇒ **安全层确实在净收益上是赚的**，不是白拿到达。
+- **对"把一步角点训准"这条训练路线的直接警告**：一步候选的多样性，恰恰来自角点的**偏差/欠缩放**（每条 ε 被削到不同的衰减幅度）。如果训练把一步映射成两段那样的收敛解，候选可能一起塌缩 ⇒ **按当前安全层口径，到达率可能不升反降**（实测就是这个方向：steps=2 开闸 −12.9 pp）。所以任何"修角点"的训练都必须**同时**处理候选多样性（更大 K、或给候选注入受控多样性），否则就是拿开环收益去换闭环到达。
+- 顺带修一处历史口径：§9.2 的"策略单独碰撞率 <3%"这一格，现在才有合法读数 **58.6%**（一步）/ **55.9%**（两段）——差距巨大，且以前从没在同一配方下报过。
+
 ## §9.2 记分板（2026-10-20 现状，最好的合法口径 = `--inc_cap 1` + K=16）

 | 判据（§9.2） | 要求 | 当前 | 差距 |