Add enum MDNodaRepository and class MDNodaLink for the overarching data

structure

src/classes/MDNodaLink.php

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+<?PHP
+/**
+ * Describes a reference to an external norm data repository.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Describes a reference to an external norm data repository.
+ */
+final class MDNodaLink {
+
+    public readonly MDNodaRepository $source;
+    public readonly string $id;
+
+    /**
+     * Constructor.
+     *
+     * @param MDNodaRepository $source Source repository.
+     * @param string           $id     ID.
+     *
+     * @return void
+     */
+    public function __construct(MDNodaRepository $source, string $id) {
+
+        $this->source = $source;
+        if (($validatedId = $this->source->validateId(trim($id))) === false) {
+            throw new MDInvalidNodaLink("Invalid noda ID / link: " . $id . " [Repository: " . $this->source->toDbName() . "]");
+        }
+        $this->id = $validatedId;
+
+    }
+}

src/classes/README.md

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+# Full-scale data types that provide further functionality but end up representing base data types