perf(kernel): P27 第二步 — Rumor 扫描去装箱 + 裁剪判定 O(1)

两条互相独立、均不改变保留集合与查询结果的优化：

1) 扫描去装箱：latestRumorFor / rumorIsDuplicate 用 `=` 比较 [<Struct>] 的
   NpcId/RumorId（含 RumorId option）会走泛型结构相等并逐条装箱（实测 ~24 B/条，
   Sim.chat 单独测得 N=16000 时 393,736 B/次、随 N 线性）。改为先解构底层
   int/int64 再比较；Parent 用 int64 option 比较替代 RumorId option 结构相等。
   语义完全等价，扫描不再逐条分配。

2) 裁剪判定 O(1)：World 新增派生镜像 RumorCount/RumorOldestDay（不参与存档，
   读档由列表经 rumorWorkingSetStats 重建）。appendRumor 用「count+1>capacity
   或 min(oldest,day)<cutoff」O(1) 判定，仅真需裁剪时才走 trimRumors；旧 consRumor
   每次追加都 rumorsNeedTrim 全表扫一遍。条件与 needTrim 逐字等价，保留集合相同。

等价性验证：5 天 2000 聊天/天跑两遍 WorldSave.save 逐字节相同，
RumorCount/RumorOldestDay 与对列表重算一致（count=8000、oldest_day=1）。

Kernel 长跑基准 30 NPC × 2000 聊天/天（--rumor-bench D 30 2000）：
 days  修复前 alloc / ms        修复后 alloc / ms
  3    930,716,456 / 1626.7     482,716,192 / 1511.3
 30  13,419,045,864 / 12182.5  4,884,067,640 / 8879.6
 60  27,297,668,608 / 23769.7  9,774,660,360 / 17078.2
60 天分配 -64%、耗时 -28%，retained_id_checksum=927964000 前后一致。

门槛：final_digest=953775FAEB2F... 三次一致、performance_determinism=PASS；
Kernel 92 / Desktop 143 全绿；--cost-probe 1/5 天 18.7s/18.8GB、90.4s/94.6GB
（全模拟分配由逐 tick NPC 快照主导，谣言子系统占比很小，故全模拟基本持平）。

3 files changed, 8 insertions, 0 deletions

diff --git a/src/LivingVillage.Kernel.Tests/RelationTests.fs b/src/LivingVillage.Kernel.Tests/RelationTests.fs
index b98252d..1a6d178 100644
--- a/src/LivingVillage.Kernel.Tests/RelationTests.fs
+++ b/src/LivingVillage.Kernel.Tests/RelationTests.fs
@@ -47,6 +47,8 @@ module RelationHarness =
           Npcs = [| mkNpc 0 mem0; mkNpc 1 mem1 |]
           Events = []
           Rumors = []
+          RumorCount = 0
+          RumorOldestDay = System.Int64.MaxValue
           Annals = [] }
 
     let chatted (tick: int64) (partner: int) : MemoryEvent =
@@ -208,6 +210,8 @@ type RelationTests () =
               Npcs = [| self; nearest; alternate |]
               Events = []
               Rumors = []
+              RumorCount = 0
+              RumorOldestDay = System.Int64.MaxValue
               Annals = [] }
         let next = Sim.step RelationHarness.zeroInput world
         match List.tryHead next.Npcs.[0].Mind.Memory with
diff --git a/src/LivingVillage.Kernel.Tests/RumorTests.fs b/src/LivingVillage.Kernel.Tests/RumorTests.fs
index 2cbb189..e8cf07a 100644
--- a/src/LivingVillage.Kernel.Tests/RumorTests.fs
+++ b/src/LivingVillage.Kernel.Tests/RumorTests.fs
@@ -52,6 +52,8 @@ module private RumorHarness =
           Npcs = Array.init count npc
           Events = []
           Rumors = []
+          RumorCount = 0
+          RumorOldestDay = System.Int64.MaxValue
           Annals = [] }
 
     let readyForChat (world: World) : World =
diff --git a/src/LivingVillage.Kernel.Tests/TradeTests.fs b/src/LivingVillage.Kernel.Tests/TradeTests.fs
index 77e5658..fa40ac3 100644
--- a/src/LivingVillage.Kernel.Tests/TradeTests.fs
+++ b/src/LivingVillage.Kernel.Tests/TradeTests.fs
@@ -56,6 +56,8 @@ module private TradeHarness =
                  npc 1 sellerMoney sellerHunger sellerFood sellerMemory None |]
           Events = []
           Rumors = []
+          RumorCount = 0
+          RumorOldestDay = System.Int64.MaxValue
           Annals = [] }
 
     let request quantity =