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/pkg/ir.go

@@ -432,6 +432,12 @@ func (e InvalidKindError) Error() string {
 // register is not safe for concurrent use. register must not be called after
 // the first instance of [Cache] has been opened.
 func register(k Kind, f IRReadFunc) {
+	openMu.Lock()
+	defer openMu.Unlock()
+
+	if opened {
+		panic("attempting to register after open")
+	}
 	if _, ok := irArtifact[k]; ok {
 		panic("attempting to register " + strconv.Itoa(int(k)) + " twice")
 	}