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, 25 insertions, 1 deletions

diff --git a/src/LivingVillage.Headless/Program.fs b/src/LivingVillage.Headless/Program.fs
index 89f4f26..7f5dcf5 100644
--- a/src/LivingVillage.Headless/Program.fs
+++ b/src/LivingVillage.Headless/Program.fs
@@ -518,6 +518,28 @@ let runCostProbe (ladder: int64 list) : int =
         Console.Out.Flush()
     0
 
+/// `--rumor-bench [days] [npcs] [chats_per_day]`：Kernel 层 Rumor 长跑基准
+/// （默认 30 NPC × 60 模拟日 × 每日 2000 次聊天）。只读、不改模拟语义。
+let runRumorBench (argv: string[]) : int =
+    let tryInt64 (index: int) =
+        if argv.Length > index then
+            match Int64.TryParse argv.[index] with
+            | true, value -> Some value
+            | _ -> None
+        else None
+    let days = tryInt64 1 |> Option.defaultValue RumorBench.defaultConfig.Days
+    let npcs = tryInt64 2 |> Option.defaultValue (int64 RumorBench.defaultConfig.NpcCount) |> int
+    let chatsPerDay = tryInt64 3 |> Option.defaultValue (int64 RumorBench.defaultConfig.ChatsPerDay) |> int
+    let config =
+        { RumorBench.defaultConfig with
+            Days = days
+            NpcCount = npcs
+            ChatsPerDay = chatsPerDay }
+    let result = RumorBench.run config
+    printfn "%s" (RumorBench.format result)
+    Console.Out.Flush()
+    0
+
 let runExisting (argv: string[]) =
     let rec parse
         (i: int)
@@ -581,7 +603,7 @@ let runExisting (argv: string[]) =
             if stats.NonFinite > 0L || stats.OutOfBounds > 0L || not replayOk then 1 else 0
     | Error msg ->
         eprintfn $"headless: {msg}"
-        eprintfn "usage: dotnet run -c Release --project src/LivingVillage.Headless -- --days N --seed S [--npc N] [--dump-relations] [--dump-rumors] [--replay-rumors] | --m5-smoke | --m6a-smoke | --batch K D | --performance-baseline | --cost-probe [D1,D2,...]"
+        eprintfn "usage: dotnet run -c Release --project src/LivingVillage.Headless -- --days N --seed S [--npc N] [--dump-relations] [--dump-rumors] [--replay-rumors] | --m5-smoke | --m6a-smoke | --batch K D | --performance-baseline | --cost-probe [D1,D2,...] | --rumor-bench [days npcs chats_per_day]"
         2
 
 let private parseCostLadder (argv: string[]) : int64 list =
@@ -608,5 +630,7 @@ let main argv =
         LivingVillage.Headless.PerformanceProbe.runDefault ()
     elif argv.Length >= 1 && argv.[0] = "--cost-probe" then
         runCostProbe (parseCostLadder argv)
+    elif argv.Length >= 1 && argv.[0] = "--rumor-bench" then
+        runRumorBench argv
     else
         runExisting argv