refactor(P4): ConvertOptions God Object → 불변 record (init-only)

mutable sealed class(13 sub-record 가변)을 불변 record로 전환 — 배치 병렬 변환(P6)에서
공유해도 안전한 스레드 안전 전제 확보 + 값 의미론. 시그니처(ConvertAsync(ConvertOptions))는 비침습 유지.

- ConvertOptions + 13개 옵션 타입(Jpeg/Png/Webp/Avif/Tiff/Bmp/Gif/PdfRender/PdfBuild/HtmlRender/
  Ocr/PdfCompress/Ai) → sealed record, 전 프로퍼티 get;init;. 8개 Provider의 READ는 무영향.
- 변이 5곳을 with 식/객체 초기화로 교정(WRITE만 깨지므로 국소적):
  MainWindow.BuildOptions(한 식으로 불변 구성), App.RunQuickAsync(with), OcrProvider(PdfRender 초기화),
  특성화 테스트 2곳, ImageOptimProviderTests.
- 카테고리 분해(RoutingOptions/OutputOptions)는 reader 파급이 커서 후속으로 보류(비침습 우선).

70개 테스트 전부 그린(골든마스터 동일성 = 동작 불변), 빌드 0경고/0오류.

src/Everything2Everything.App/App.xaml.cs

@@ -107,8 +107,7 @@ public partial class App : Application
 
         try
         {
-            var options = ConvertOptions.Quick();
-            options.VideoPreferGpu = Settings.Get("video.gpu") != "false";
+            var options = ConvertOptions.Quick() with { VideoPreferGpu = Settings.Get("video.gpu") != "false" };
             var reporter = new Progress<ConvertProgress>(p => progress.Report(p));
             var results = await Engine.ConvertManyAsync(files, outputExtension, options, reporter);
 

src/Everything2Everything.App/Views/MainWindow.xaml.cs

@@ -278,38 +278,35 @@ public partial class MainWindow : Wpf.Ui.Controls.FluentWindow
 
     private ConvertOptions BuildOptions()
     {
-        var opts = new ConvertOptions
-        {
-            OnCollision = _conflictRule,
-        };
-        opts.Jpeg.Quality = (int)QualitySlider.Value;
-        opts.Webp.Quality = (int)QualitySlider.Value;
-        opts.Avif.Quality = Math.Clamp((int)QualitySlider.Value - 30, 1, 100);
+        var q = (int)QualitySlider.Value;
 
         var custom = OutputPathTextBox.Text?.Trim();
-        if (!string.IsNullOrEmpty(custom))
-        {
-            opts.OutputLocation = OutputLocation.Custom;
-            opts.CustomOutputDirectory = custom;
-        }
-        else
-        {
-            opts.OutputLocation = OutputLocation.SubfolderBesideSource;
-        }
+        var hasCustom = !string.IsNullOrEmpty(custom);
 
         // AI 작업 종류 (txt↔txt / md↔md 등 텍스트 변환 시 LlmProvider가 사용)
-        opts.Ai.Task = (AiTaskCombo?.SelectedIndex ?? 0) switch
+        var aiTask = (AiTaskCombo?.SelectedIndex ?? 0) switch
         {
             1 => "translate",
             2 => "proofread",
             _ => "summarize",
         };
         var targetLang = AiTargetLangBox?.Text?.Trim();
-        opts.Ai.TargetLanguage = string.IsNullOrEmpty(targetLang) ? null : targetLang;
 
-        opts.VideoPreferGpu = _settings.Get("video.gpu") != "false";
-
-        return opts;
+        return new ConvertOptions
+        {
+            OnCollision = _conflictRule,
+            OutputLocation = hasCustom ? OutputLocation.Custom : OutputLocation.SubfolderBesideSource,
+            CustomOutputDirectory = hasCustom ? custom : null,
+            Jpeg = new JpegEncodingOptions { Quality = q },
+            Webp = new WebpEncodingOptions { Quality = q },
+            Avif = new AvifEncodingOptions { Quality = Math.Clamp(q - 30, 1, 100) },
+            Ai = new AiOptions
+            {
+                Task = aiTask,
+                TargetLanguage = string.IsNullOrEmpty(targetLang) ? null : targetLang,
+            },
+            VideoPreferGpu = _settings.Get("video.gpu") != "false",
+        };
     }
 
     // ============== Process queue ==============