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）。

5 files changed, 70 insertions, 14 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 =
diff --git a/src/LivingVillage.Kernel/Sim.fs b/src/LivingVillage.Kernel/Sim.fs
index cedadfa..5c4e107 100644
--- a/src/LivingVillage.Kernel/Sim.fs
+++ b/src/LivingVillage.Kernel/Sim.fs
@@ -192,6 +192,12 @@ module Sim =
           Npcs: Npc[]
           Events: InteractionEvent list
           Rumors: RumorEvent list
+          /// 派生镜像：工作集条数（= Rumors.Length）。不参与存档序列化，读档由列表重建；
+          /// 用于把每追加一条都要全表扫描的裁剪判定降为 O(1)。
+          RumorCount: int
+          /// 派生镜像：工作集内最小 DayIndex（空表为 Int64.MaxValue）。语义等价于
+          /// `rumorsNeedTrim` 的「存在早于保留窗口的条目」判定。
+          RumorOldestDay: int64
           Annals: AnnalEntry list }
 
     type DialogueFailure =
@@ -481,6 +487,8 @@ module Sim =
           Npcs = [| npc |]
           Events = []
           Rumors = []
+          RumorCount = 0
+          RumorOldestDay = System.Int64.MaxValue
           Annals = [] }
 
     let initialWorldN (seed: uint64) (count: int) : World =
@@ -680,19 +688,49 @@ module Sim =
         else
             rumors
 
-    /// 追加一条谣言并顺手裁剪工作集；容量内且新鲜时等价于 `rumor :: rumors`。
-    let private consRumor (nowTick: int64) (rumor: RumorEvent) (rumors: RumorEvent list) : RumorEvent list =
-        trimRumors nowTick (rumor :: rumors)
+    /// 工作集派生统计：(条数, 最小 DayIndex；空表为 Int64.MaxValue)。仅裁剪/读档等非热路径调用。
+    let rumorWorkingSetStats (rumors: RumorEvent list) : int * int64 =
+        rumors.Length,
+        (rumors |> List.fold (fun acc rumor -> min acc rumor.DayIndex) System.Int64.MaxValue)
+
+    let private oldestRumorDay (rumors: RumorEvent list) : int64 = snd (rumorWorkingSetStats rumors)
+
+    /// 追加一条谣言并维护派生元数据。容量内且新鲜时等价于 `rumor :: rumors`，且**不再全表扫描**
+    /// 判断是否需要裁剪（旧 `consRumor` 每次追加都会 `rumorsNeedTrim` 走一遍工作集）；
+    /// 仅当条数超容量或存在早于保留窗口的条目时才走 `trimRumors`，保留集合与旧实现逐条相同。
+    let private appendRumor (nowTick: int64) (rumor: RumorEvent) (world: World) : World =
+        let cutoffDay = max 0L (rumorDayIndex nowTick - rumorRetentionDays)
+        let nextCount = world.RumorCount + 1
+        let nextOldest = min world.RumorOldestDay rumor.DayIndex
+        if nextCount > rumorCapacity || nextOldest < cutoffDay then
+            let trimmed = trimRumors nowTick (rumor :: world.Rumors)
+            { world with
+                Rumors = trimmed
+                RumorCount = trimmed.Length
+                RumorOldestDay = oldestRumorDay trimmed }
+        else
+            { world with
+                Rumors = rumor :: world.Rumors
+                RumorCount = nextCount
+                RumorOldestDay = nextOldest }
+
+    /// `NpcId`/`RumorId` 是 `[<Struct>]` 单例判别联合：直接用 `=` 比较会走泛型结构相等并
+    /// 在每次比较时为两侧装箱（实测 ~24 B/次）。在按 tick 全表扫描的热路径里逐条比较，
+    /// 这是 Rumor 分配的主要来源。这里改为先解构出底层 int/int64 再比较（语义完全等价，
+    /// 仅去掉装箱），扫描不再产生逐条分配。
+    let inline private npcIdValue (NpcId id) : int = id
+    let inline private rumorIdValue (RumorId id) : int64 = id
 
     let private latestRumorFor (receiver: NpcId) (world: World) : RumorEvent option =
         // 工作集最新在前且 Tick 非递增：第一条命中的即“Tick 最大、Id 最大”，与原实现
         // （filter 后 sortByDescending 取头）等价；窗口外无分配提前返回。
+        let receiverId = npcIdValue receiver
         let rec loop (rest: RumorEvent list) : RumorEvent option =
             match rest with
             | [] -> None
             | rumor :: tail ->
                 if world.Tick - rumor.Tick > rumorFreshnessTicks then None
-                elif rumor.Receiver = receiver
+                elif npcIdValue rumor.Receiver = receiverId
                      && rumor.Tick <= world.Tick
                      && rumorStrengthAt world.Tick rumor >= rumorMinimumStrength then Some rumor
                 else loop tail
@@ -783,11 +821,13 @@ module Sim =
                 let avatarMemory = recordMemory world.Tick (Dialogue(target, intent, response)) world.Avatar.Mind.Memory
                 let event = { Tick = world.Tick; Kind = DialogueEvent outcome }
                 let updated =
-                    { world with
-                        Avatar = { world.Avatar with Mind = { world.Avatar.Mind with Memory = avatarMemory } }
-                        Npcs = newNpcs
-                        Events = world.Events @ [ event ]
-                        Rumors = consRumor world.Tick rumor world.Rumors }
+                    appendRumor
+                        world.Tick
+                        rumor
+                        { world with
+                            Avatar = { world.Avatar with Mind = { world.Avatar.Mind with Memory = avatarMemory } }
+                            Npcs = newNpcs
+                            Events = world.Events @ [ event ] }
                 let withRumorAnnal =
                     appendAnnal
                         { Tick = world.Tick
@@ -826,16 +866,21 @@ module Sim =
               RecentMemory = List.truncate 8 npc.Mind.Memory })
 
     let private rumorIsDuplicate (request: ChatRequest) (parent: RumorEvent option) (world: World) : bool =
-        let parentId = parent |> Option.map (fun rumor -> rumor.Id)
+        let parentId = parent |> Option.map (fun rumor -> rumorIdValue rumor.Id)
+        let narratorId = npcIdValue request.Narrator
+        let receiverId = npcIdValue request.Receiver
         // 窗口外条目强度必 < rumorMinimumStrength，与原全表 exists 等价；命中或越窗即无分配返回。
+        // `Parent` 用底层 int64 比较而非 `RumorId option` 结构相等，避免逐条装箱。
         let rec loop (rest: RumorEvent list) : bool =
             match rest with
             | [] -> false
             | rumor :: tail ->
                 if world.Tick - rumor.Tick > rumorFreshnessTicks then false
-                elif rumor.Narrator = request.Narrator
-                     && rumor.Receiver = request.Receiver
-                     && rumor.Parent = parentId
+                elif npcIdValue rumor.Narrator = narratorId
+                     && npcIdValue rumor.Receiver = receiverId
+                     && (match rumor.Parent with
+                         | Some parentRumor -> parentId = Some(rumorIdValue parentRumor)
+                         | None -> parentId.IsNone)
                      && rumor.Tick <= world.Tick
                      && rumorStrengthAt world.Tick rumor >= rumorMinimumStrength then true
                 else loop tail
@@ -936,7 +981,7 @@ module Sim =
                                 { npc with
                                     Mind = { npc.Mind with Memory = recordMemory world.Tick (Rumor rumor.Id) npc.Mind.Memory } }
                             { chatted with Npcs = newNpcs }
-                    let updated = { withReceiverRumor with Rumors = consRumor world.Tick rumor world.Rumors }
+                    let updated = appendRumor world.Tick rumor withReceiverRumor
                     ChatSucceeded(
                         Some rumor,
                         appendAnnal
diff --git a/src/LivingVillage.Kernel/WorldSave.fs b/src/LivingVillage.Kernel/WorldSave.fs
index f1b0be3..b3bdfa9 100644
--- a/src/LivingVillage.Kernel/WorldSave.fs
+++ b/src/LivingVillage.Kernel/WorldSave.fs
@@ -645,6 +645,7 @@ module WorldSave =
                 let events = List.init eventCount (fun index -> readInteraction reader (sprintf "event[%d]" index))
                 let rumorCount = readCount reader "rumors"
                 let rumors = List.init rumorCount (fun index -> readRumor reader (sprintf "rumor[%d]" index))
+                let rumorStats = rumorWorkingSetStats rumors
                 let annalCount = readCount reader "annals"
                 let annals = List.init annalCount (fun index -> readAnnal reader (sprintf "annal[%d]" index))
                 let todayTask =
@@ -667,6 +668,8 @@ module WorldSave =
                        Npcs = npcs
                        Events = events
                        Rumors = rumors
+                       RumorCount = fst rumorStats
+                       RumorOldestDay = snd rumorStats
                        Annals = annals },
                      occupation)
             with