internal/pkg: reserve kind range

This is useful for custom implementations of Artifact.

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

internal/pkg/pkg.go

@@ -191,6 +191,10 @@ const (
 	KindExecNet
 	// KindFile is the kind of [Artifact] returned by [NewFile].
 	KindFile
+
+	// KindCustomOffset is the first [Kind] value reserved for implementations
+	// not from this package.
+	KindCustomOffset = 1 << 31
 )
 
 // Ident returns a deterministic identifier for the supplied params and