internal/pkg: record cache variant on-disk

This makes custom artifacts much less error-prone to use.

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

internal/rosa/rosa.go

@@ -14,6 +14,12 @@ import (
 	"hakurei.app/internal/pkg"
 )
 
+// Extension is the variant identification string of custom artifact
+// implementations registered by package rosa.
+const Extension = "rosa"
+
+func init() { pkg.SetExtension(Extension) }
+
 const (
 	// kindEtc is the kind of [pkg.Artifact] of cureEtc.
 	kindEtc = iota + pkg.KindCustomOffset