WIP: the mouse editor hands wasm its captured seed (improvise-ixxx)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

harness/tests/contract.spec.js

@@ -287,6 +287,39 @@ test.describe("a pointer event reaches the client and its answer is painted", ()
     await expect(page.locator("#grid tbody td.selected")).toHaveCount(1);
   });
 
+  // improvise-ixxx. The editor hands wasm the seed it captured at open, and
+  // only wasm decides an untouched commit writes nothing — JS must pass it.
+  test("an untouched mouse editor sends no write, and a touched one does", async ({
+    page,
+  }) => {
+    await page.addInitScript(() => {
+      window.__posted = [];
+      const post = Worker.prototype.postMessage;
+      Worker.prototype.postMessage = function (message, ...rest) {
+        window.__posted.push(String(message));
+        return post.call(this, message, ...rest);
+      };
+    });
+    await openApp(page);
+    const cells = page.locator("#grid tbody td");
+    const writes = () =>
+      page.evaluate(() => window.__posted.filter((m) => m.includes("SetCell")).length);
+
+    await cells.nth(4).click();
+    await expect(cells.nth(4).locator("input")).toHaveCount(1);
+    await page.keyboard.press("Enter");
+    await expect(page.locator("#grid input")).toHaveCount(0);
+    await page.waitForTimeout(300);
+    expect(await writes()).toBe(0);
+
+    // The control: without it a page that never sends anything passes above.
+    await cells.nth(4).click();
+    await expect(cells.nth(4).locator("input")).toHaveCount(1);
+    await page.keyboard.type("1");
+    await page.keyboard.press("Enter");
+    await expect.poll(writes).toBe(1);
+  });
+
   // improvise-b8ot moved these five surfaces onto one handler per stable root.
   // The three tests below are the properties a naive delegation loses, each of
   // which is silent: the page still paints, and only the pointer stops working.