perf(kernel): P27 第一步 — Rumor 长跑基准与修复前量化

新增 Kernel 层 Rumor 长跑基准（RumorBench.fs + headless --rumor-bench
[days npcs chats_per_day]），把「聊天→生成/追溯谣言」路径从 Sim.step 的 NPC
快照分配解耦，隔离测量谣言工作集随世界年龄的分配/耗时；同 Config 的
retained_id_checksum 证明确定性。

复核 M3-1 前提：World.Rumors 已被 M6a（d94ec2d/cc850bf）按 3 天保留窗口收敛，
单日成本平稳（1/5/10 天 ≈18.7/18.1/19.8s、≈19GB/天），超线性项消失。
剩余 Rumor 成本 = 每次聊天的两条 O(N) 扫描 latestRumorFor/rumorIsDuplicate，
且 [<Struct>] NpcId/RumorId 用 = 比较会逐条装箱（Sim.chat 单独测得 N=16000
时 393,736 B/次、随 N 线性）。

修复前基准（30 NPC，--rumor-bench D 30 2000）：
 3d  chat_calls=6000   retained=6000  alloc=930,716,456   ms=1626.7
 30d chat_calls=60000  retained=8000  alloc=13,419,045,864 ms=12182.5
 60d chat_calls=120000 retained=8000  alloc=27,297,668,608 ms=23769.7
（60d 工作集稳定 8000，不随年龄增长。）

docs/维护说明.md 新增 P27 小节记录上述现状与量级。不改任何模拟语义。

1 files changed, 109 insertions, 0 deletions

diff --git a/src/LivingVillage.Kernel/RumorBench.fs b/src/LivingVillage.Kernel/RumorBench.fs
new file mode 100644
index 0000000..145f92c
--- /dev/null
+++ b/src/LivingVillage.Kernel/RumorBench.fs
@@ -0,0 +1,109 @@
+namespace LivingVillage.Kernel
+
+open System
+open System.Diagnostics
+open LivingVillage.Kernel.Sim
+
+/// Kernel 层 Rumor 长跑基准：把「聊天 → 生成/追溯传闻」这条路径单独拎出来，按
+/// 模拟日推进 tick 并注入固定节奏的聊天，长期测量 Rumor 工作集随世界年龄的
+/// 分配与耗时量级（与 `Sim.step` 的 NPC 快照分配解耦，便于隔离谣言子系统）。
+///
+/// 该基准只读、不改变任何模拟语义；所有输入由 `Config` 决定，无墙钟/`System.Random`，
+/// 同一 `Config` 两次运行产生相同的工作集校验和。
+module RumorBench =
+
+    type Config =
+        { Seed: uint64
+          NpcCount: int
+          Days: int64
+          /// 每个模拟日注入的聊天次数（决定谣言产生速率）。
+          ChatsPerDay: int }
+
+    type Result =
+        { Seed: uint64
+          NpcCount: int
+          Days: int64
+          ChatsPerDay: int
+          ChatCalls: int64
+          RumorsCreated: int64
+          RumorsRetained: int
+          /// 保留工作集内各条目 Id 之和，用于证明同一 Config 的确定性。
+          RetainedIdChecksum: int64
+          AllocatedBytes: int64
+          ElapsedMs: float }
+
+    /// 任务默认口径：30 NPC × 60 模拟日 × 每日 2000 次聊天。
+    let defaultConfig : Config =
+        { Seed = 42UL
+          NpcCount = 30
+          Days = 60L
+          ChatsPerDay = 2000 }
+
+    /// 测试/快速核验口径（同样是 30 NPC，但只跑 3 天）。
+    let quickConfig : Config =
+        { defaultConfig with Days = 3L }
+
+    let private resetParticipants (ids: NpcId list) (world: World) : World =
+        let npcs = Array.copy world.Npcs
+        for id in ids do
+            let (NpcId index) = id
+            if index >= 0 && index < npcs.Length then
+                npcs.[index] <-
+                    { npcs.[index] with
+                        Mind = { npcs.[index].Mind with Action = Wander } }
+        { world with Npcs = npcs }
+
+    let private retainedChecksum (rumors: RumorEvent list) : int64 =
+        rumors |> List.sumBy (fun rumor -> let (RumorId id) = rumor.Id in id)
+
+    let run (config: Config) : Result =
+        if config.NpcCount < 2 then invalidArg (nameof config) "NpcCount must be at least 2"
+        if config.Days <= 0L then invalidArg (nameof config) "Days must be positive"
+        if config.ChatsPerDay <= 0 then invalidArg (nameof config) "ChatsPerDay must be positive"
+        if config.ChatsPerDay > 1000000 then invalidArg (nameof config) "ChatsPerDay is too large"
+        let callsPerDay = int64 config.ChatsPerDay
+        let ticksPerCall = max 1L (ticksPerDay / callsPerDay)
+        let totalCalls = config.Days * callsPerDay
+        let mutable world = Sim.initialWorldN config.Seed config.NpcCount
+        let mutable created = 0L
+        let mutable calls = 0L
+        GC.Collect(2, GCCollectionMode.Forced, true, true)
+        GC.WaitForPendingFinalizers()
+        GC.Collect(2, GCCollectionMode.Forced, true, true)
+        let allocatedBefore = GC.GetAllocatedBytesForCurrentThread()
+        let stopwatch = Stopwatch.StartNew()
+        for i in 0L .. totalCalls - 1L do
+            let dayIndex = i / callsPerDay
+            let within = i % callsPerDay
+            let tick = dayIndex * ticksPerDay + within * ticksPerCall
+            let narratorIndex = int (i % int64 config.NpcCount)
+            let receiverIndex =
+                int ((i + 1L + (i / int64 config.NpcCount) % int64 (config.NpcCount - 1)) % int64 config.NpcCount)
+            let narrator = NpcId narratorIndex
+            let receiver = NpcId receiverIndex
+            world <-
+                resetParticipants [ narrator; receiver ] { world with Tick = tick }
+            match Sim.chat { Narrator = narrator; Receiver = receiver } world with
+            | ChatSucceeded (rumor, next) ->
+                world <- next
+                calls <- calls + 1L
+                if rumor.IsSome then created <- created + 1L
+            | ChatRejected _ -> ()
+        stopwatch.Stop()
+        let allocatedAfter = GC.GetAllocatedBytesForCurrentThread()
+        { Seed = config.Seed
+          NpcCount = config.NpcCount
+          Days = config.Days
+          ChatsPerDay = config.ChatsPerDay
+          ChatCalls = calls
+          RumorsCreated = created
+          RumorsRetained = world.Rumors.Length
+          RetainedIdChecksum = retainedChecksum world.Rumors
+          AllocatedBytes = allocatedAfter - allocatedBefore
+          ElapsedMs = stopwatch.Elapsed.TotalMilliseconds }
+
+    let format (result: Result) : string =
+        sprintf
+            "rumor_bench seed=%d npcs=%d days=%d chats_per_day=%d chat_calls=%d rumors_created=%d rumors_retained=%d retained_id_checksum=%d allocated_bytes=%d elapsed_ms=%.3f"
+            result.Seed result.NpcCount result.Days result.ChatsPerDay result.ChatCalls result.RumorsCreated
+            result.RumorsRetained result.RetainedIdChecksum result.AllocatedBytes result.ElapsedMs