internal/pkg: decompress zstd streams

This uses internal/zstd until compress/zstd becomes available.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/compress.go

@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"io"
 	"os"
+
+	"hakurei.app/internal/zstd"
 )
 
 const (
@@ -13,6 +15,8 @@ const (
 	Gzip = iota
 	// Bzip2 denotes a stream compressed via [bzip2].
 	Bzip2
+	// Zstd denotes a stream compressed via [zstd].
+	Zstd
 )
 
 // A decompressArtifact is a [FileArtifact] decompressing a backing
@@ -117,6 +121,12 @@ func (a *decompressArtifact) Cure(r *RContext) (io.ReadCloser, error) {
 			io.Closer
 		}{bzip2.NewReader(br), br}, nil
 
+	case Zstd:
+		return struct {
+			io.Reader
+			io.Closer
+		}{zstd.NewReader(br), br}, nil
+
 	default:
 		return nil, os.ErrInvalid
 	}