perf: gate perf logs and dedup build_config_update on first message (#8627)

Signed-off-by: Bradley Axen <baxen@squareup.com>

ui/goose2/src/shared/lib/perfLog.ts

@@ -0,0 +1,39 @@
+/**
+ * Gated performance logger for frontend timing instrumentation.
+ *
+ * Enabled when any of the following is true:
+ *   - Running under Vite dev (`import.meta.env.DEV`)
+ *   - `localStorage.getItem("goose.perf") === "1"`
+ *
+ * Otherwise a no-op, so perf call sites add zero runtime cost in release
+ * builds for users who have not opted in.
+ *
+ * Messages are prefixed with `[perf:<channel>]` by callers; this helper
+ * is intentionally dumb and forwards the already-formatted string.
+ */
+function isEnabled(): boolean {
+  try {
+    if (import.meta.env?.DEV) return true;
+  } catch {
+    // import.meta may be unavailable in some test contexts
+  }
+  try {
+    if (
+      typeof localStorage !== "undefined" &&
+      localStorage.getItem("goose.perf") === "1"
+    ) {
+      return true;
+    }
+  } catch {
+    // localStorage can throw in restricted contexts
+  }
+  return false;
+}
+
+const enabled = isEnabled();
+
+export function perfLog(message: string): void {
+  if (!enabled) return;
+  // eslint-disable-next-line no-console
+  console.log(message);
+}