feat(kernel): P31 step2 story stage state machine

- Occupation.Story: three-segment arc per occupation (accept task -> visit
  households -> conclude), driven one stage per player dialogue with day
  gates derived from the same splitmix pattern as the daily task pool
  (start day = hash(seed ^ storySeed, day 0) mod 3 + 2, range [2,4]).
- relationMeanToPlayer: decayed valence mean over acquainted NPCs only
  (same decay formula as Sim.relationMatrix); no acquaintances = 0.
- Three endings (warm/plain/distant) chosen by deterministic thresholds
  (>=0.3 / >=0.0 / <0); ending text is fixed at the concluding dialogue.
- onDialogue returns (state, summary text option) for the caller to write
  into the yearbook; StoryStage itself is unchanged (0..3) and already
  roundtrips through the existing v3 occupation segment.
- 14 new kernel tests (101 -> 115); performance digest unchanged.

1 files changed, 103 insertions, 0 deletions

diff --git a/src/LivingVillage.Kernel/Occupation.fs b/src/LivingVillage.Kernel/Occupation.fs
index 8ce5060..00cd2e9 100644
--- a/src/LivingVillage.Kernel/Occupation.fs
+++ b/src/LivingVillage.Kernel/Occupation.fs
@@ -272,3 +272,106 @@ module Occupation =
         match kind with
         | None -> baseResponse
         | Some kind -> remapResponse kind intent baseResponse
+
+    // ---- 轻剧情线（design §5 / P-d）：三段式 接任务 → 走访 → 收束 ----
+    // 纯年鉴/对话驱动：每次玩家对话至多推进一段，产出只有（StoryStage 推进 + 年鉴摘要文本），
+    // 由调用方写入世界。全部输入 = (seed, 世界 tick, 关系状态, 职业状态)，纯函数、确定性。
+    // 开线日沿用每日任务的 (seed, profession, day) splitmix 哈希模式。
+    module Story =
+
+        type Ending =
+            | Warm
+            | Plain
+            | Distant
+
+        /// 剧情线种子：与每日任务种子同源异值，两两不撞（见测试）。
+        let storySeedOf (kind: Kind) : uint64 =
+            match kind with
+            | Farmer -> 0x5555555555555555UL
+            | Fisher -> 0x6666666666666666UL
+            | Peddler -> 0x7777777777777777UL
+            | Scholar -> 0x8888888888888888UL
+
+        let lineNameOf (kind: Kind) : string =
+            match kind with
+            | Farmer -> "雨水与年成"
+            | Fisher -> "水位与收成"
+            | Peddler -> "新货的路子"
+            | Scholar -> "庙前的争论"
+
+        /// StoryStage 语义：0 未开始 / 1 已接任务 / 2 走访中 / 3 已收束。
+        let stageNameOf (stage: int) : string =
+            match stage with
+            | 1 -> "已接任务"
+            | 2 -> "走访中"
+            | 3 -> "已收束"
+            | _ -> "未开始"
+
+        /// 开线日 = hash(seed ^ storySeed, day 0) mod 3 + 2，落在 [2, 4]。
+        let startDayOf (seed: uint64) (kind: Kind) : int64 =
+            (int64 (taskHash seed (storySeedOf kind) 0L % 3UL)) + 2L
+
+        /// 玩家与村民的关系均值：与 Sim.relationMatrix 同口径（衰减 valence），
+        /// 但只统计认识玩家的村民（有 Chatted/Dialogue 记忆者），无一认识 = 0（中性）。
+        let relationMeanToPlayer (world: Sim.World) : float32 =
+            let player = Sim.playerId
+            let mutable total = 0.0f
+            let mutable known = 0
+            for npc in world.Npcs do
+                let mutable valence = 0.0f
+                let mutable touched = false
+                for e in npc.Mind.Memory do
+                    if e.Tick <= world.Tick then
+                        let involvesPlayer =
+                            match e.Kind with
+                            | Sim.Chatted partner -> partner = player
+                            | Sim.Dialogue (actor, _, _) -> actor = player
+                            | _ -> false
+                        if involvesPlayer then
+                            touched <- true
+                            valence <- valence + e.Valence * Sim.relationDecayWeight world.Tick e.Tick
+                if touched then
+                    known <- known + 1
+                    total <- total + valence
+            if known = 0 then 0.0f else total / float32 known
+
+        /// 结局分档：均值 ≥ 0.3 热络；[0, 0.3) 平常；< 0 淡漠。纯阈值，无随机。
+        let endingOf (mean: float32) : Ending =
+            if mean >= 0.3f then Warm
+            elif mean >= 0.0f then Plain
+            else Distant
+
+        let endingTextOf (kind: Kind) (ending: Ending) : string =
+            match kind, ending with
+            | Farmer, Warm -> "雨水与年成有了结果：乡邻都说雨水匀，今年差不了。"
+            | Farmer, Plain -> "雨水与年成有了结果：雨水尚可，年成将就，照旧耕作。"
+            | Farmer, Distant -> "雨水与年成有了结果：问不出准信，年成只能看天。"
+            | Fisher, Warm -> "水位与收成有了结果：乡邻说得真切，水位稳了，收成坏不了。"
+            | Fisher, Plain -> "水位与收成有了结果：众说平常，水位尚可，收成将就。"
+            | Fisher, Distant -> "水位与收成有了结果：问了几家都淡淡的，水情还得自己看。"
+            | Peddler, Warm -> "新货的路子有了结果：乡邻热心指路，新货不愁卖。"
+            | Peddler, Plain -> "新货的路子有了结果：说法平常，先小量试卖。"
+            | Peddler, Distant -> "新货的路子有了结果：各家都淡淡的，路子还得自己闯。"
+            | Scholar, Warm -> "庙前的争论有了结果：各说各理，事有分晓。"
+            | Scholar, Plain -> "庙前的争论有了结果：众说纷纭，暂无定论。"
+            | Scholar, Distant -> "庙前的争论有了结果：无人理会，先记在册上。"
+
+        /// 用一次玩家对话推进剧情线：同输入两次结果一致。
+        /// 返回（新状态, 应写入年鉴的摘要文本 option）；无门控命中时原样返回。
+        /// 结局文本在收束瞬间按当时关系均值定格，由调用方连同世界存档（年鉴快照）持久化。
+        let onDialogue (seed: uint64) (state: State) (world: Sim.World) : State * string option =
+            let kind = state.Profile.Kind
+            let day = dayIndexOf world.Tick
+            let startDay = startDayOf seed kind
+            match state.StoryStage with
+            | 0 when day >= startDay ->
+                { state with StoryStage = 1 },
+                Some (sprintf "接了件事：%s，得去村里多问问。" (lineNameOf kind))
+            | 1 when day >= startDay + 1L ->
+                { state with StoryStage = 2 },
+                Some "走访了几户人家，改日再问。"
+            | 2 when day >= startDay + 2L ->
+                let ending = endingOf (relationMeanToPlayer world)
+                { state with StoryStage = 3 },
+                Some (endingTextOf kind ending)
+            | _ -> state, None