fix(bench): rebuild each round's level set from its runs, not from a snapshot

The round comparison was about to be wrong. levels/campaign-50-r2.json was
copied from levels/campaign-50.json to preserve "the set round 2 played", but
select-50 writes that path in place and several selection experiments ran
between the round and the copy. The file kept 28 of the 50 boards round 2 had
actually played and 46 of round 3's — so the comparison was scoring one round
against another round's levels, and it read as round 3 regressing.

Runs store the whole board and its campaign index, so a round is its own record
and cannot drift. round-set.mjs rebuilds it, the report uses that, and the
snapshot file is gone rather than left lying around looking authoritative.

With the rounds correctly matched, round 3 is not a regression and not an
improvement: back band 2.17 against round 2's 2.39, on a rating that retests at
0.55, which is not enough to separate seventeen levels. The report says that
rather than picking whichever reading flatters the change.

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

bench/board.json

@@ -125,13 +125,13 @@
      "id": "T1.5",
      "title": "扩产到 ≥120 候选并去重、家族均衡",
      "status": "done",
-     "note": "扫了 480 + 240 个种子，得 684 个候选；过两道新闸门后 228 关。扩产的目的是付得起闸门的代价，不是数量本身"
+     "note": "两批共 720 个种子 → 1113 个候选 → 过闸门 369 关（原 228）。家族 weave 262 / switchback 58 / orbits 39 / spiral 10"
     },
     {
      "id": "T1.6",
      "title": "终局三枚互锁（hero+gate+tail）",
      "status": "blocked",
-     "note": "仍是深关卡的瓶颈：684 个候选里深度≥4 的只有 67 个，过闸门后剩 16 个——后档只能做到平均 4.1 转，是供给限制不是设计选择"
+     "note": "量化了：扫 1113 个候选，过闸门后深度≥4 的只有 23 个（2%）。第二次扩池把种子数翻倍，深关卡只从 16 涨到 23 —— 难度上限是构造的供给瓶颈，不是筛选问题。后档平均 4.0 转就是这个供给能支撑的上限"
     },
     {
      "id": "T1.7",
@@ -259,6 +259,10 @@
   {
    "at": "R2 结果",
    "text": "难度评分按档 2.00/2.05/2.40（R1 是三档全平），无解感 1.21/1.15/1.74。意外把每关跑了两次，得到 45 对复测：84% 重放了完全相同的动作序列，机械指标 r=1.00，难度评分 r=0.55——且评分分歧的那几对全是同轨迹。结论：agent 近乎确定性，同一关不必跑两遍"
+  },
+  {
+   "at": "供给瓶颈",
+   "text": "扩池两轮共 720 种子、1113 候选，深度≥4 的过闸门后只有 23 个（2%）。种子翻倍只让深关卡从 16 涨到 23，说明 growDeepLevel 难以造出需要四转以上的关卡——这是后档难度的硬上限"
   }
  ]
 }