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/parse-concurrency.ts

@@ -0,0 +1,17 @@
+import * as os from "node:os";
+
+export function parseConcurrency(): number {
+    const args = process.argv.slice(2);
+    for (let i = 0; i < args.length; i++) {
+        if (
+            (args[i] === "--concurrency" || args[i] === "-j") &&
+            i + 1 < args.length
+        ) {
+            const n = parseInt(args[i + 1], 10);
+            if (!isNaN(n) && n > 0) {
+                return n;
+            }
+        }
+    }
+    return os.cpus().length;
+}