feat: adapt roster sources into package resolution

Launcher.Next.Infrastructure/FileSystemCatalogPackageAcquirer.cs

@@ -0,0 +1,32 @@
+using Launcher.Next.Core;
+
+namespace Launcher.Next.Infrastructure;
+
+/// <summary>
+/// Reads a catalog descriptor only. The returned source is later copied into
+/// local immutable staging by <see cref="IPackageSynchronizer"/>; it is never
+/// an active launch root.
+/// </summary>
+public sealed class FileSystemCatalogPackageAcquirer(string catalogRoot) : IPackageCatalogPackageAcquirer
+{
+    public async Task<CatalogPackage?> AcquireAsync(PackageRequirement requirement, CancellationToken cancellationToken)
+    {
+        var root = Path.GetFullPath(catalogRoot);
+        var candidate = Path.GetFullPath(Path.Combine(root, requirement.PackageId, requirement.RequiredVersion));
+        if (!IsWithin(root, candidate) || !Directory.Exists(candidate) || IsReparsePoint(candidate))
+            return null;
+
+        var manifestPath = Path.Combine(candidate, "package.manifest.json");
+        if (!File.Exists(manifestPath) || IsReparsePoint(manifestPath))
+            return null;
+
+        var manifest = PackageManifestJson.Parse(await File.ReadAllTextAsync(manifestPath, cancellationToken));
+        return new CatalogPackage(manifest, candidate);
+    }
+
+    private static bool IsWithin(string root, string candidate) =>
+        candidate.StartsWith(Path.TrimEndingDirectorySeparator(root) + Path.DirectorySeparatorChar, StringComparison.OrdinalIgnoreCase);
+
+    private static bool IsReparsePoint(string path) =>
+        (File.GetAttributes(path) & FileAttributes.ReparsePoint) != 0;
+}