refactor: simplify and add memory indexing

src/index/unique.ts

@@ -0,0 +1,20 @@
+import type { PrimaryKey } from "./primary.ts";
+
+export class UniqueIndex {
+  readonly #index = new Map<string, PrimaryKey>();
+
+  insert(value: any, pk: PrimaryKey) {
+    if (this.#index.has(value)) {
+      throw new Error(`Unique constraint violation: ${value}`);
+    }
+    this.#index.set(value, pk);
+  }
+
+  lookup(value: any): PrimaryKey | undefined {
+    return this.#index.get(value);
+  }
+
+  delete(value: any) {
+    this.#index.delete(value);
+  }
+}