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/prefixed-logger.ts

@@ -0,0 +1,28 @@
+import { Logger } from "sync-client";
+
+export class PrefixedLogger extends Logger {
+    private readonly base: Logger;
+    private readonly prefix: string;
+
+    public constructor(base: Logger, prefix: string) {
+        super();
+        this.base = base;
+        this.prefix = prefix;
+    }
+
+    public override debug(message: string): void {
+        this.base.debug(`[${this.prefix}] ${message}`);
+    }
+
+    public override info(message: string): void {
+        this.base.info(`[${this.prefix}] ${message}`);
+    }
+
+    public override warn(message: string): void {
+        this.base.warn(`[${this.prefix}] ${message}`);
+    }
+
+    public override error(message: string): void {
+        this.base.error(`[${this.prefix}] ${message}`);
+    }
+}