refactor: simplify and add memory indexing

src/index/primary.ts

@@ -0,0 +1,24 @@
+import type { AnyDocument } from "../types.ts";
+
+export type PrimaryKey = string;
+
+export class PrimaryIndex<TSchema extends AnyDocument> {
+  readonly #index = new Map<PrimaryKey, TSchema>();
+
+  constructor(readonly key: string) {}
+
+  insert(pk: PrimaryKey, document: TSchema) {
+    if (this.#index.has(pk)) {
+      throw new Error(`Duplicate primary key: ${pk}`);
+    }
+    this.#index.set(pk, document);
+  }
+
+  get(pk: PrimaryKey): TSchema | undefined {
+    return this.#index.get(pk);
+  }
+
+  delete(pk: PrimaryKey) {
+    this.#index.delete(pk);
+  }
+}