refactor: Source Generator 自动生成 Metadata，消除 FormatPlugin/FormatMetadata 重复声明

- FormatPluginAttribute 新增 IsBinary 可选命名参数
- Source Generator 自动推断 SupportsImport/SupportsExport/IsBinary
- Source Generator 为每个格式类生成 partial class 的 Metadata 实现
- 83 个格式类改为 partial，删除手写的 Metadata 声明
- 信息单一来源：所有元数据仅在 [FormatPlugin] 属性中声明一次

重构前（每个格式类需要声明两次）：
  [FormatPlugin("rime", "Rime", 150)]
  public sealed class RimeImporter : TextFormatImporter {
      public override FormatMetadata Metadata { get; } = new("rime", "Rime", 150, ...);
  }

重构后（信息只声明一次）：
  [FormatPlugin("rime", "Rime", 150)]
  public sealed partial class RimeImporter : TextFormatImporter { }

Author: studyzy

src/ImeWlConverter.Abstractions/FormatPluginAttribute.cs

@@ -2,6 +2,7 @@ namespace ImeWlConverter.Abstractions;
 
 /// <summary>
 /// Marks a class as an IME format plugin for compile-time registration via Source Generator.
+/// The Source Generator reads these values to auto-generate the <c>Metadata</c> property.
 /// </summary>
 [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
 public sealed class FormatPluginAttribute : Attribute
@@ -15,6 +16,9 @@ public sealed class FormatPluginAttribute : Attribute
     /// <summary>Sort order for UI display.</summary>
     public int SortOrder { get; }
 
+    /// <summary>Whether this format uses binary encoding (auto-detected from base class if not set).</summary>
+    public bool IsBinary { get; set; }
+
     /// <summary>
     /// Initializes a new instance of the <see cref="FormatPluginAttribute"/> class.
     /// </summary>

src/ImeWlConverter.Formats/BaiduBcd/BaiduBcdImporter.cs

@@ -9,16 +9,12 @@ namespace ImeWlConverter.Formats.BaiduBcd;
 
 /// <summary>Baidu Shouji bcd dictionary importer (binary, delegates to legacy).</summary>
 [FormatPlugin("bcd", "百度手机bcd", 1020)]
-public sealed class BaiduBcdImporter : BinaryFormatImporter
+public sealed partial class BaiduBcdImporter : BinaryFormatImporter
 {
     static BaiduBcdImporter()
     {
         Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
     }
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bcd", "百度手机bcd", 1020, SupportsImport: true, SupportsExport: false, IsBinary: true);
-
     protected override IReadOnlyList<WordEntry> ParseBinary(Stream input, CancellationToken ct)
     {
         var tempFile = Path.GetTempFileName();

src/ImeWlConverter.Formats/BaiduBdict/BaiduBdictImporter.cs

@@ -9,16 +9,12 @@ namespace ImeWlConverter.Formats.BaiduBdict;
 
 /// <summary>Baidu Pinyin bdict dictionary importer (binary, delegates to legacy).</summary>
 [FormatPlugin("bdict", "百度拼音bdict", 100)]
-public sealed class BaiduBdictImporter : BinaryFormatImporter
+public sealed partial class BaiduBdictImporter : BinaryFormatImporter
 {
     static BaiduBdictImporter()
     {
         Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
     }
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdict", "百度拼音bdict", 100, SupportsImport: true, SupportsExport: false, IsBinary: true);
-
     protected override IReadOnlyList<WordEntry> ParseBinary(Stream input, CancellationToken ct)
     {
         var tempFile = Path.GetTempFileName();

src/ImeWlConverter.Formats/BaiduPinyin/BaiduPinyinExporter.cs

@@ -7,13 +7,9 @@ namespace ImeWlConverter.Formats.BaiduPinyin;
 
 /// <summary>Baidu Pinyin dictionary exporter (text format). Supports both Chinese and English entries.</summary>
 [FormatPlugin("bdpy", "百度拼音", 90)]
-public sealed class BaiduPinyinExporter : TextFormatExporter
+public sealed partial class BaiduPinyinExporter : TextFormatExporter
 {
     protected override Encoding FileEncoding => Encoding.Unicode;
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdpy", "百度拼音", 90, SupportsImport: false, SupportsExport: true);
-
     protected override string? FormatEntry(WordEntry entry)
     {
         if (entry.IsEnglish)

src/ImeWlConverter.Formats/BaiduPinyin/BaiduPinyinImporter.cs

@@ -8,13 +8,9 @@ namespace ImeWlConverter.Formats.BaiduPinyin;
 
 /// <summary>Baidu Pinyin dictionary importer (text format). Supports both Chinese and English entries.</summary>
 [FormatPlugin("bdpy", "百度拼音", 90)]
-public sealed class BaiduPinyinImporter : TextFormatImporter
+public sealed partial class BaiduPinyinImporter : TextFormatImporter
 {
     protected override Encoding FileEncoding => Encoding.Unicode;
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdpy", "百度拼音", 90, SupportsImport: true, SupportsExport: false);
-
     protected override IEnumerable<WordEntry> ParseLine(string line)
     {
         var array = line.Split('\t');

src/ImeWlConverter.Formats/BaiduPinyinBackup/BaiduPinyinBackupImporter.cs

@@ -9,16 +9,12 @@ namespace ImeWlConverter.Formats.BaiduPinyinBackup;
 
 /// <summary>Baidu Pinyin backup file importer (binary, delegates to legacy).</summary>
 [FormatPlugin("bdpybin", "百度拼音备份", 20)]
-public sealed class BaiduPinyinBackupImporter : BinaryFormatImporter
+public sealed partial class BaiduPinyinBackupImporter : BinaryFormatImporter
 {
     static BaiduPinyinBackupImporter()
     {
         Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
     }
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdpybin", "百度拼音备份", 20, SupportsImport: true, SupportsExport: false, IsBinary: true);
-
     protected override IReadOnlyList<WordEntry> ParseBinary(Stream input, CancellationToken ct)
     {
         var tempFile = Path.GetTempFileName();

src/ImeWlConverter.Formats/BaiduShouji/BaiduShoujiExporter.cs

@@ -7,13 +7,9 @@ namespace ImeWlConverter.Formats.BaiduShouji;
 
 /// <summary>Baidu Mobile dictionary exporter. Format: word(pin|yin) 20000</summary>
 [FormatPlugin("bdsj", "百度手机", 1000)]
-public sealed class BaiduShoujiExporter : TextFormatExporter
+public sealed partial class BaiduShoujiExporter : TextFormatExporter
 {
     protected override Encoding FileEncoding => Encoding.Unicode;
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdsj", "百度手机", 1000, SupportsImport: false, SupportsExport: true);
-
     protected override string? FormatEntry(WordEntry entry)
     {
         var pinyin = entry.Code?.GetPrimaryCode("|") ?? "";

src/ImeWlConverter.Formats/BaiduShouji/BaiduShoujiImporter.cs

@@ -8,13 +8,9 @@ namespace ImeWlConverter.Formats.BaiduShouji;
 
 /// <summary>Baidu Mobile dictionary importer. Format: word(pin|yin) rank</summary>
 [FormatPlugin("bdsj", "百度手机", 1000)]
-public sealed class BaiduShoujiImporter : TextFormatImporter
+public sealed partial class BaiduShoujiImporter : TextFormatImporter
 {
     protected override Encoding FileEncoding => Encoding.Unicode;
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdsj", "百度手机", 1000, SupportsImport: true, SupportsExport: false);
-
     protected override IEnumerable<WordEntry> ParseLine(string line)
     {
         var parenIdx = line.IndexOf('(');

src/ImeWlConverter.Formats/BaiduShoujiEng/BaiduShoujiEngExporter.cs

@@ -7,13 +7,9 @@ namespace ImeWlConverter.Formats.BaiduShoujiEng;
 
 /// <summary>Baidu Mobile English dictionary exporter. Format: word\t(54999+rank)</summary>
 [FormatPlugin("bdsje", "百度手机英文", 1010)]
-public sealed class BaiduShoujiEngExporter : TextFormatExporter
+public sealed partial class BaiduShoujiEngExporter : TextFormatExporter
 {
     protected override Encoding FileEncoding => Encoding.ASCII;
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdsje", "百度手机英文", 1010, SupportsImport: false, SupportsExport: true);
-
     protected override string? FormatEntry(WordEntry entry)
     {
         return $"{entry.Word}\t{54999 + entry.Rank}";

src/ImeWlConverter.Formats/BaiduShoujiEng/BaiduShoujiEngImporter.cs

@@ -8,13 +8,9 @@ namespace ImeWlConverter.Formats.BaiduShoujiEng;
 
 /// <summary>Baidu Mobile English dictionary importer. Format: word\trank</summary>
 [FormatPlugin("bdsje", "百度手机英文", 1010)]
-public sealed class BaiduShoujiEngImporter : TextFormatImporter
+public sealed partial class BaiduShoujiEngImporter : TextFormatImporter
 {
     protected override Encoding FileEncoding => Encoding.ASCII;
-
-    public override FormatMetadata Metadata { get; } = new(
-        "bdsje", "百度手机英文", 1010, SupportsImport: true, SupportsExport: false);
-
     protected override IEnumerable<WordEntry> ParseLine(string line)
     {
         var parts = line.Split('\t');