feat(desktop): 目标上方浮动情境提示

新增 FloatingPrompt 纯布局模块（含 measureWidth 与 PixelText 等宽规则）：短提示锚定在唯一目标头顶上方，水平居中并钳制在视口内，避开左下 HUD 带（height-160）与右上状态条带（宽度 248）；沿用现有 CJK 图集字体路径，不新造字形；对话/需求面板与菜单打开时不显示（复用 worldInputAllowed 判定与渲染门）。回归测试覆盖 1280x720/1920x1080 五个极限锚点、宽度测量、面板开启时提示为空且禁用世界输入。

3 files changed, 116 insertions, 4 deletions

diff --git a/src/LivingVillage.Desktop.Tests/PrototypeTests.fs b/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
index af34802..7e4ce5f 100644
--- a/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
+++ b/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
@@ -630,6 +630,46 @@ type PrototypeTests () =
         CollectionAssert.AreEqual(snapshot, tieWorld.Npcs)
 
     [<TestMethod>]
+    member _.FloatingPromptStaysInViewportAndClearOfHudBands () =
+        for (width, height) in [ (1280, 720); (1920, 1080) ] do
+            let anchors : (float32 * float32) list =
+                [ (100.0f, 100.0f)      // near top-left
+                  (float32 width / 2.0f, float32 height / 2.0f)
+                  (float32 width - 60.0f, 120.0f)   // near top-right strip
+                  (200.0f, float32 height - 200.0f) // above bottom-left HUD band
+                  (float32 width - 100.0f, float32 height - 120.0f) ]
+            for (anchorX, anchorY) in anchors do
+                for text in [ "E 交谈"; "E 进屋"; "E 出屋" ] do
+                    let rect = FloatingPrompt.layout width height anchorX anchorY 0.0f 0.0f 2 text
+                    let textWidth = FloatingPrompt.measureWidth text 2
+                    Assert.AreEqual<int>(textWidth, rect.Width)
+                    Assert.IsTrue(rect.Width > 0, "prompt must not be truncated to zero width")
+                    Assert.IsTrue(rect.X >= 0 && rect.X + rect.Width <= width, sprintf "x overflow %A %A" (anchorX, anchorY) rect)
+                    Assert.IsTrue(rect.Y >= 0 && rect.Y + rect.Height <= height, sprintf "y overflow %A %A" (anchorX, anchorY) rect)
+                    // Never cover the bottom-left HUD band (camera 0: band starts at height - 160).
+                    Assert.IsTrue(rect.Y + rect.Height <= height - FloatingPrompt.hudBottomBandHeight + 8, sprintf "bottom HUD overlap %A %A" (anchorX, anchorY) rect)
+                    // Never cover the top-right status strip.
+                    if rect.X + rect.Width >= width - FloatingPrompt.hudTopRightWidth then
+                        Assert.IsTrue(rect.Y >= FloatingPrompt.hudTopRightHeight, sprintf "top-right HUD overlap %A %A" (anchorX, anchorY) rect)
+
+    [<TestMethod>]
+    member _.FloatingPromptMeasureMatchesPixelTextAdvances () =
+        // E(12) space(8) 交(16) 谈(16) at scale 2.
+        Assert.AreEqual<int>(52, FloatingPrompt.measureWidth "E 交谈" 2)
+        Assert.AreEqual<int>(20, FloatingPrompt.measureWidth "E " 2)
+
+    [<TestMethod>]
+    member _.FloatingPromptHiddenWhilePanelsOrMenusAreOpen () =
+        let world = Sim.initialWorldN 42UL 2
+        let dialogueView = { M5Interaction.initial with Panel = DialoguePanel }
+        let cleared = M5Interaction.refreshPrompt world dialogueView
+        Assert.AreEqual<string option>(None, cleared.Prompt)
+        Assert.IsFalse(M5Interaction.worldInputAllowed dialogueView)
+
+        let needsView = { M5Interaction.initial with Panel = NeedsPanel }
+        Assert.AreEqual<string option>(None, M5Interaction.refreshPrompt world needsView |> fun value -> value.Prompt)
+
+    [<TestMethod>]
     member _.WorldPanelDoesNotRenderA永久KeyList () =
         let world = Sim.initialWorldN 42UL 0
         let lines = M5Interaction.panelLines world M5Interaction.initial
diff --git a/src/LivingVillage.Desktop/Game.fs b/src/LivingVillage.Desktop/Game.fs
index 4720008..1db5c3d 100644
--- a/src/LivingVillage.Desktop/Game.fs
+++ b/src/LivingVillage.Desktop/Game.fs
@@ -666,6 +666,19 @@ type LivingVillageGame() as this =
                 let spec = VillageArt.npcSpriteSpecAtTarget npc.Id npc.Pos npc.Mind.Target world.Tick
                 VillageArt.drawCharacter spriteBatch artTextures camera npc.Pos spec profile.WorldTint
         spriteBatch.End()
+
+        // Contextual floating prompt over the unique target; same CJK atlas font path as the HUD.
+        if m5View.Panel = M5Panel.WorldPanel && menu.Page = Playing then
+            match m5View.Prompt, m5View.PromptTargetPos with
+            | Some text, Some targetPos ->
+                let scale = 2
+                let rect = FloatingPrompt.layout viewport.Width viewport.Height targetPos.X targetPos.Y camera.X camera.Y scale text
+                spriteBatch.Begin()
+                spriteBatch.Draw(pixel, Rectangle(rect.X - 6, rect.Y - 5, rect.Width + 12, rect.Height + 8), Color(15, 19, 25, 200))
+                spriteBatch.Draw(pixel, Rectangle(rect.X - 6, rect.Y - 5, rect.Width + 12, 2), Color(92, 180, 190))
+                PixelText.draw spriteBatch pixel cjkAtlas rect.X rect.Y scale (Color(240, 250, 240)) text
+                spriteBatch.End()
+            | _ -> ()
         this.DrawM5Overlay()
 
     member private this.DrawM5Overlay() =
diff --git a/src/LivingVillage.Desktop/Interaction.fs b/src/LivingVillage.Desktop/Interaction.fs
index 916544c..3c011cb 100644
--- a/src/LivingVillage.Desktop/Interaction.fs
+++ b/src/LivingVillage.Desktop/Interaction.fs
@@ -45,7 +45,8 @@ type M5View =
       Status: string
       StatusTick: int64 option
       HomeMode: HomeMode
-      Prompt: string option }
+      Prompt: string option
+      PromptTargetPos: Vec2 option }
 
 module InteractionResolver =
 
@@ -149,6 +150,52 @@ module InteractionResolver =
 
     let resolveAction = resolve
 
+module FloatingPrompt =
+
+    type PromptRect =
+        { X: int
+          Y: int
+          Width: int
+          Height: int }
+
+    // HUD exclusion geometry shared with the Game.fs renderer: bottom-left lightweight
+    // panel band and top-right corner status strip.
+    let hudBottomBandHeight = 160
+    let hudTopRightWidth = 248
+    let hudTopRightHeight = 72
+
+    /// Width of text as PixelText.draw advances it (scale applied, CJK 8px, latin 6px, space 4px).
+    let measureWidth (text: string) (scale: int) : int =
+        text.ToUpperInvariant()
+        |> Seq.fold (fun cursor character ->
+            if character = ' ' then cursor + 4 * scale
+            elif CjkGlyphAtlas.sourceRectangle character |> Option.isSome then cursor + 8 * scale
+            else cursor + 6 * scale) 0
+
+    /// Anchor the one short prompt above the unique target: horizontally centered on the
+    /// entity, clamped inside the viewport, and never covering either HUD band.
+    let layout
+        (viewportWidth: int)
+        (viewportHeight: int)
+        (anchorWorldX: float32)
+        (anchorWorldY: float32)
+        (cameraX: float32)
+        (cameraY: float32)
+        (scale: int)
+        (text: string) : PromptRect =
+        let width = measureWidth text scale
+        let height = 8 * scale + 10
+        let margin = 8
+        let screenX = int anchorWorldX - int cameraX - width / 2
+        let screenY = int anchorWorldY - int cameraY - 56
+        let x = 
+            if width + margin * 2 > viewportWidth then margin
+            else max margin (min (viewportWidth - margin - width) screenX)
+        let topSafe = if x + width >= viewportWidth - hudTopRightWidth then hudTopRightHeight + margin else margin
+        let bottomSafe = viewportHeight - hudBottomBandHeight - height - margin
+        let y = max topSafe (min bottomSafe screenY)
+        { X = x; Y = y; Width = width; Height = height }
+
 module M5Interaction =
 
     let initial : M5View =
@@ -160,16 +207,20 @@ module M5Interaction =
           Status = "ready"
           StatusTick = None
           HomeMode = Outside
-          Prompt = None }
+          Prompt = None
+          PromptTargetPos = None }
 
     let worldInputAllowed (view: M5View) : bool =
         view.Panel = WorldPanel
 
     let refreshPrompt (world: World) (view: M5View) : M5View =
         if worldInputAllowed view then
-            { view with Prompt = InteractionResolver.resolve world view.HomeMode |> Option.map (fun target -> target.Text) }
+            let resolved = InteractionResolver.resolve world view.HomeMode
+            { view with
+                Prompt = resolved |> Option.map (fun target -> target.Text)
+                PromptTargetPos = resolved |> Option.map (fun target -> target.Position) }
         else
-            { view with Prompt = None }
+            { view with Prompt = None; PromptTargetPos = None }
 
     let resolveAction (world: World) (homeMode: HomeMode) : InteractionTarget option =
         InteractionResolver.resolveAction world homeMode
@@ -250,6 +301,7 @@ module M5Interaction =
                         Menu = None
                         Chronicle = Sim.annalText next
                         Prompt = None
+                        PromptTargetPos = None
                         Status = sprintf "dialogue intent=%A response=%A" outcome.Intent outcome.Response
                         StatusTick = Some next.Tick }
                 | DialogueRejected(failure, next) ->
@@ -258,6 +310,7 @@ module M5Interaction =
                         Panel = WorldPanel
                         Menu = None
                         Prompt = None
+                        PromptTargetPos = None
                         Status = sprintf "dialogue rejected: %s" (dialogueFailureName failure)
                         StatusTick = Some next.Tick }
 
@@ -286,6 +339,7 @@ module M5Interaction =
                     Panel = DialoguePanel
                     Menu = Some menu
                     Prompt = None
+                    PromptTargetPos = None
                     Status = menuStatus menu }
             | Some { Action = EnterHome _ } ->
                 let pixel = { PixelPosition.X = world.Avatar.Pos.X; Y = world.Avatar.Pos.Y }
@@ -298,6 +352,7 @@ module M5Interaction =
                         Menu = None
                         HomeMode = nextMode
                         Prompt = None
+                        PromptTargetPos = None
                         Status = "home entered" }
                 | Error failure -> world, { view with Prompt = None; Status = sprintf "home entry rejected: %A" failure }
             | Some { Action = ExitHome _ } ->
@@ -309,6 +364,7 @@ module M5Interaction =
                         Menu = None
                         HomeMode = nextMode
                         Prompt = None
+                        PromptTargetPos = None
                         Status = "home exited" }
                 | Error failure -> world, { view with Prompt = None; Status = sprintf "home exit rejected: %A" failure }
             | None ->
@@ -327,6 +383,7 @@ module M5Interaction =
                     Panel = ObservationPanel
                     Observations = observations
                     Prompt = None
+                    PromptTargetPos = None
                     Status = sprintf "observation visible_npcs=%d" observations.Length }
         | ToggleNeeds ->
             if view.Panel = NeedsPanel then
@@ -337,6 +394,7 @@ module M5Interaction =
                     Panel = NeedsPanel
                     Needs = Some(Sim.needsPanel world)
                     Prompt = None
+                    PromptTargetPos = None
                     Status = "needs panel open" }
         | ShowChronicle ->
             world,
@@ -344,6 +402,7 @@ module M5Interaction =
                 Panel = ChroniclePanel
                 Chronicle = Sim.annalText world
                 Prompt = None
+                PromptTargetPos = None
                 Status = sprintf "chronicle entries=%d" world.Annals.Length }
         | ClosePanel -> world, refreshPrompt world { view with Panel = WorldPanel; Menu = None; Status = "panel closed" }