core: manual buffer sync with sync-rows, reuse unchanged buffers and pipelines across activations

Also lets the screenshot suite point at another reference directory and settle longer.

Amp-Thread-ID: https://ampcode.com/threads/T-01a0b2dd-db32-712d-99ce-12ce4a86d979
Co-authored-by: Heaust Azure <heaust.azure@gmail.com>

core/index.js

@@ -113,6 +113,12 @@ export class YawnCore {
     return this.#request("delete-object", { name, id });
   }
 
+  /** Uploads rows `first .. first + count` of `name` to every active GPU buffer bound to that array. */
+  async syncRows(name, first, count) {
+    await this.ready;
+    return this.#request("sync-rows", { name, first, count });
+  }
+
   async compileGraph(serialized) {
     await this.ready;
     return this.#request("compile-graph", { serialized });