Add deterministic-tests workspace

Scripted multi-client harness against a real server (~110 scenario
tests, server-control, managed-websocket, test-runner). Wires the new
package into frontend/package.json workspaces and the lint script.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/deterministic-tests/src/tests/update-during-create-processing.test.ts

@@ -0,0 +1,42 @@
+import type { AssertableState } from "../utils/assertable-state";
+import type { TestDefinition } from "../test-definition";
+
+export const updateDuringCreateProcessingTest: TestDefinition = {
+    description:
+        "Client 0 creates a file while the server is paused, then immediately updates it. After the server resumes, both clients should converge with the updated content.",
+    clients: 2,
+    steps: [
+        { type: "enable-sync", client: 0 },
+        { type: "enable-sync", client: 1 },
+        { type: "barrier" },
+
+        { type: "pause-server" },
+
+        {
+            type: "create",
+            client: 0,
+            path: "file.md",
+            content: "initial"
+        },
+
+        {
+            type: "update",
+            client: 0,
+            path: "file.md",
+            content: "updated during create"
+        },
+
+        { type: "resume-server" },
+        { type: "barrier" },
+
+        {
+            type: "assert-consistent",
+            verify: (s: AssertableState): void => {
+                s.assertFileCount(1).assertContent(
+                    "file.md",
+                    "updated during create"
+                );
+            }
+        }
+    ]
+};