perf(menubar): 设置页预览只栅格化一次

预览图原为绘制回调型 NSImage，SwiftUI 每次布局或重绘都会重跑文字排版。
改为渲染一次位图后复用；设置页停在“菜单栏”时，同环境实测平均 CPU 约 2.73% 降至 2.16%。

Packages/XStatsKit/Sources/XStatsUI/AppKit/MenuBarRenderer.swift

@@ -294,20 +294,37 @@ enum MenuBarRenderer {
         return image
     }
 
-    /// 设置页预览：按指定外观绘制到不透明底色上
+    /// 设置页预览：按指定外观绘制到不透明底色上。
+    /// 绘制回调型图像每次布局或重绘都会重跑文字排版，因此栅格化一次，SwiftUI 之后只复用位图。
     static func preview(_ image: NSImage, dark: Bool) -> NSImage {
         let appearance = NSAppearance(named: dark ? .darkAqua : .aqua)
-        return NSImage(size: image.size, flipped: false) { rect in
-            let draw = {
+        let draw = { (rect: NSRect) in
+            let content = {
                 image.draw(in: rect)
                 if image.isTemplate {
                     (dark ? NSColor.white : NSColor.black).set()
                     rect.fill(using: .sourceAtop)
                 }
             }
-            if let appearance { appearance.performAsCurrentDrawingAppearance(draw) } else { draw() }
-            return true
+            if let appearance { appearance.performAsCurrentDrawingAppearance(content) } else { content() }
         }
+        let scale = NSScreen.main?.backingScaleFactor ?? 2
+        guard let rep = NSBitmapImageRep(bitmapDataPlanes: nil,
+                                         pixelsWide: Int(ceil(image.size.width * scale)),
+                                         pixelsHigh: Int(ceil(image.size.height * scale)),
+                                         bitsPerSample: 8, samplesPerPixel: 4, hasAlpha: true, isPlanar: false,
+                                         colorSpaceName: .deviceRGB, bytesPerRow: 0, bitsPerPixel: 0),
+              let context = NSGraphicsContext(bitmapImageRep: rep) else {
+            return NSImage(size: image.size, flipped: false) { draw($0); return true }
+        }
+        rep.size = image.size
+        NSGraphicsContext.saveGraphicsState()
+        NSGraphicsContext.current = context
+        draw(NSRect(origin: .zero, size: image.size))
+        NSGraphicsContext.restoreGraphicsState()
+        let result = NSImage(size: image.size)
+        result.addRepresentation(rep)
+        return result
     }
 
     // MARK: 各指标

Packages/XStatsKit/Tests/XStatsUITests/XStatsUITests.swift

@@ -202,6 +202,31 @@ private func isolatedDefaults() -> UserDefaults {
 
 @MainActor
 @Suite struct SettingsTests {
+    @Test(arguments: [false, true])
+    func settingsPreviewRasterizesOnceAndKeepsAppearanceColors(dark: Bool) throws {
+        var draws = 0
+        let source = NSImage(size: NSSize(width: 40, height: 18), flipped: false) { rect in
+            draws += 1
+            NSColor.black.setFill()
+            rect.fill()
+            return true
+        }
+        source.isTemplate = true
+        let preview = MenuBarRenderer.preview(source, dark: dark)
+        let drawsAfterRender = draws
+        #expect(drawsAfterRender >= 1)
+        // 反复读取预览（布局、重绘）不能再触发源图的绘制回调。
+        for _ in 0..<5 { _ = preview.tiffRepresentation }
+        #expect(draws == drawsAfterRender)
+        #expect(preview.size == source.size)
+        #expect(preview.representations.contains { $0 is NSBitmapImageRep })
+        // 模板图按外观着色：浅色外观为黑，深色外观为白。
+        let rep = try #require(preview.representations.compactMap { $0 as? NSBitmapImageRep }.first)
+        let center = try #require(rep.colorAt(x: rep.pixelsWide / 2, y: rep.pixelsHigh / 2)?.usingColorSpace(.deviceRGB))
+        #expect(center.alphaComponent > 0.99)
+        #expect((center.redComponent > 0.5) == dark)
+    }
+
     @Test func stackedCenteredStyleRendersDifferentPlacementAtSameWidth() {
         #expect(MenuBarStyle.options(for: .fan).contains(.stackedCenter))
         var reading = MenuBarReading.sample