manifest: add optional format field for release dispatch

introduces an optional top-level "format" field on the signed manifest,
defaulting to legacy-json-blob when absent so existing installs keep
parsing unchanged. follow-up commits wire the release format factory
and the m2pack strategy against this value.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Metin2Launcher/Manifest/Manifest.cs

@@ -10,6 +10,17 @@ namespace Metin2Launcher.Manifest;
 /// </summary>
 public sealed class Manifest
 {
+    [JsonPropertyName("format")]
+    public string? Format { get; set; }
+
+    /// <summary>
+    /// Effective release format. Defaults to <c>legacy-json-blob</c> when the
+    /// field is absent, which preserves backward compatibility with manifests
+    /// produced before the m2pack migration.
+    /// </summary>
+    public string EffectiveFormat =>
+        string.IsNullOrWhiteSpace(Format) ? "legacy-json-blob" : Format;
+
     [JsonPropertyName("version")]
     public string Version { get; set; } = "";