feat(p41): 夜帧船篷对比暗化

- VillageArt.drawCc0BoatOverlay 增加夜层专用暗化系数 0.7（仅夜光晕层生效，
  白天不调用 overlay，p40-day/p40-boat 逐字节不变）
- P40ArtTests 新增 NightBoatAwningIsAtLeast40GreyDarkerThanSurroundingGlow：
  内嵌 PNG 解码读取夜帧，断言船 2 瓦包围盒比 24px 光晕环至少暗 40 灰阶
- 新增 scripts/check-p40-night-boat-contrast.py 独立像素守卫
- 重出 docs/evidence/p40-night.png（contrast 103.2 >= 40）

2 files changed, 111 insertions, 2 deletions

diff --git a/src/LivingVillage.Desktop.Tests/P40ArtTests.fs b/src/LivingVillage.Desktop.Tests/P40ArtTests.fs
index 15e6601..da7becf 100644
--- a/src/LivingVillage.Desktop.Tests/P40ArtTests.fs
+++ b/src/LivingVillage.Desktop.Tests/P40ArtTests.fs
@@ -64,6 +64,73 @@ type P40ArtTests () =
             || (r, g, b) = (28, 38, 22)
             || (r, g, b) = (22, 30, 17))
 
+    // --- P41 boat night contrast: pixel regression on the recorded night frame ---
+    // Decodes the MonoGame 8-bit RGBA PNG (color type 6, may use filters 0-4) so the
+    // test can measure the rendered frame without a GPU.
+    let decodePng (path: string) : int * int * (int -> int -> (int * int * int * int)) =
+        let bytes = System.IO.File.ReadAllBytes path
+        let be32 (o: int) =
+            (int bytes.[o] <<< 24) ||| (int bytes.[o + 1] <<< 16) ||| (int bytes.[o + 2] <<< 8) ||| int bytes.[o + 3]
+        let width = be32 16
+        let height = be32 20
+        let mutable offset = 8
+        let mutable idat = new System.IO.MemoryStream()
+        while offset < bytes.Length do
+            let length = be32 offset
+            let tag = System.Text.Encoding.ASCII.GetString(bytes, offset + 4, 4)
+            if tag = "IDAT" then idat.Write(bytes, offset + 8, length)
+            offset <- offset + 12 + length
+        use z = new System.IO.Compression.ZLibStream(new System.IO.MemoryStream(idat.ToArray()), System.IO.Compression.CompressionMode.Decompress)
+        use ms = new System.IO.MemoryStream()
+        z.CopyTo ms
+        let raw = ms.ToArray()
+        let bpp = 4
+        let stride = width * bpp
+        let pixels = Array.zeroCreate<byte> (stride * height)
+        let paeth (a: int) (b: int) (c: int) =
+            let p = a + b - c
+            let pa = abs (p - a)
+            let pb = abs (p - b)
+            let pc = abs (p - c)
+            if pa <= pb && pa <= pc then a elif pb <= pc then b else c
+        for y in 0 .. height - 1 do
+            let ft = int raw.[y * (stride + 1)]
+            let rowIn = y * (stride + 1) + 1
+            let rowOut = y * stride
+            let prev = rowOut - stride
+            for x in 0 .. stride - 1 do
+                let rv = int raw.[rowIn + x]
+                let a = if x >= bpp then int pixels.[rowOut + x - bpp] else 0
+                let b = if y > 0 then int pixels.[prev + x] else 0
+                let c = if y > 0 && x >= bpp then int pixels.[prev + x - bpp] else 0
+                let v =
+                    match ft with
+                    | 0 -> rv
+                    | 1 -> rv + a
+                    | 2 -> rv + b
+                    | 3 -> rv + (a + b) / 2
+                    | 4 -> rv + paeth a b c
+                    | _ -> failwithf "unsupported PNG filter %d" ft
+                pixels.[rowOut + x] <- byte (v &&& 0xFF)
+        let get x y =
+            let i = (y * width + x) * 4
+            (int pixels.[i], int pixels.[i + 1], int pixels.[i + 2], int pixels.[i + 3])
+        width, height, get
+
+    let evidencePath (name: string) : string option =
+        let mutable dir = Some (new System.IO.DirectoryInfo(System.AppContext.BaseDirectory))
+        let mutable found = None
+        while found.IsNone && dir.IsSome do
+            let d = dir.Value
+            let candidate = System.IO.Path.Combine(d.FullName, "docs/evidence", name)
+            if System.IO.File.Exists candidate then found <- Some candidate
+            else dir <- Option.ofObj d.Parent
+        found
+
+    let luma (r: int, g: int, b: int, _: int) =
+        0.299 * float r + 0.587 * float g + 0.114 * float b
+
+
     [<TestMethod>]
     member _.PavingCoversTrafficNetworkAndDropsIsolatedScatter () =
         let paving = cc0PavingTiles map
@@ -168,3 +235,37 @@ type P40ArtTests () =
             topRow (cc0TileIndexBoatLeft * 32) 31,
             topRow (cc0TileIndexBoatRight * 32) 0,
             "left/right awning halves must meet at the same arch height (no broken arc)")
+
+    [<TestMethod>]
+    member _.NightBoatAwningIsAtLeast40GreyDarkerThanSurroundingGlow () =
+        match evidencePath "p40-night.png" with
+        | None -> Assert.Inconclusive "docs/evidence/p40-night.png has not been recorded"
+        | Some path ->
+            let _, _, pixel = decodePng path
+            // The p40-night hook centers tile (234,50) in a 1280x720 viewport; the
+            // 2-tile boat occupies tiles 234-235 at row 50 (Sim.tilePixels = 32).
+            let camX = 234 * 32 + 16 - 640
+            let camY = 50 * 32 + 16 - 360
+            let x0 = 234 * 32 - camX
+            let x1 = 235 * 32 + 32 - camX
+            let y0 = 50 * 32 - camY
+            let y1 = 51 * 32 - camY
+            let mutable boatSum = 0.0
+            let mutable boatN = 0
+            for y in y0 .. y1 - 1 do
+                for x in x0 .. x1 - 1 do
+                    boatSum <- boatSum + luma (pixel x y)
+                    boatN <- boatN + 1
+            let m = 24
+            let mutable ringSum = 0.0
+            let mutable ringN = 0
+            for y in y0 - m .. y1 - 1 + m do
+                for x in x0 - m .. x1 - 1 + m do
+                    if not (x >= x0 && x < x1 && y >= y0 && y < y1) then
+                        ringSum <- ringSum + luma (pixel x y)
+                        ringN <- ringN + 1
+            let boat = boatSum / float boatN
+            let ring = ringSum / float ringN
+            Assert.IsTrue(
+                ring - boat >= 40.0,
+                sprintf "night boat awning contrast %.1f grey is below the 40 grey budget (boat %.1f ring %.1f)" (ring - boat) boat ring)
diff --git a/src/LivingVillage.Desktop/VillageArt.fs b/src/LivingVillage.Desktop/VillageArt.fs
index 2b2b0e7..0f176aa 100644
--- a/src/LivingVillage.Desktop/VillageArt.fs
+++ b/src/LivingVillage.Desktop/VillageArt.fs
@@ -970,18 +970,26 @@ module VillageArt =
                 for (rect, color) in lanternGlowLayers centerX centerY radiusPixels 8 255 214 150 peakAlpha do
                     spriteBatch.Draw(pixel, rect, color)
 
+    let cc0BoatNightTintScale = 0.7f
+
     let drawCc0BoatOverlay
         (spriteBatch: SpriteBatch)
         (textures: ArtTextures)
         (camera: XnaVector2)
         (map: MapGen.Result)
         (tint: XnaColor) =
+        let night =
+            XnaColor(
+                int (float32 tint.R * cc0BoatNightTintScale),
+                int (float32 tint.G * cc0BoatNightTintScale),
+                int (float32 tint.B * cc0BoatNightTintScale),
+                int tint.A)
         match textures.Cc0TileAtlas with
         | Some cc0 ->
             match cc0BoatTile map with
             | Some (boatX, boatY) ->
-                drawCc0Tile spriteBatch cc0 tint (cc0TileRect cc0TileIndexBoatLeft) (boatX * Sim.tilePixels - int camera.X) (boatY * Sim.tilePixels - int camera.Y)
-                drawCc0Tile spriteBatch cc0 tint (cc0TileRect cc0TileIndexBoatRight) ((boatX + 1) * Sim.tilePixels - int camera.X) (boatY * Sim.tilePixels - int camera.Y)
+                drawCc0Tile spriteBatch cc0 night (cc0TileRect cc0TileIndexBoatLeft) (boatX * Sim.tilePixels - int camera.X) (boatY * Sim.tilePixels - int camera.Y)
+                drawCc0Tile spriteBatch cc0 night (cc0TileRect cc0TileIndexBoatRight) ((boatX + 1) * Sim.tilePixels - int camera.X) (boatY * Sim.tilePixels - int camera.Y)
             | None -> ()
         | None -> ()