feat: v0.9.0 — single-screen 3-pane shell + left Assets pane (wallet/reputation/fleet/time controls) from a single /api/state poll; right pane placeholder

lib/useGameState.ts

@@ -0,0 +1,55 @@
+'use client';
+
+import { useCallback, useEffect, useRef, useState } from 'react';
+import type { StateSnapshot } from './game/state';
+
+export interface GameStateResult {
+  state: StateSnapshot | null;
+  // True until the first successful fetch (stale-while-error afterwards).
+  loading: boolean;
+  error: string | null;
+  // Trigger an immediate refetch (used after POST /api/sim so the UI updates without waiting for the next tick).
+  refresh: () => void;
+}
+
+// Single fetch of the full game-state snapshot. Exported separately from the hook so it can be unit-tested with a mocked fetch.
+export async function fetchState(): Promise<StateSnapshot> {
+  const res = await fetch('/api/state', { cache: 'no-store' });
+  if (!res.ok) throw new Error(`GET /api/state → HTTP ${res.status}`);
+  return (await res.json()) as StateSnapshot;
+}
+
+// The shell's single polling source (v0.9.0): one GET /api/state loop for the whole page,
+// so every pane renders from the same snapshot instead of running its own poll loops.
+export function useGameState(intervalMs = 1000): GameStateResult {
+  const [state, setState] = useState<StateSnapshot | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+  // Overlap guard: if the previous request is still in flight (slow server), skip this tick.
+  const inFlightRef = useRef(false);
+
+  const refresh = useCallback(() => {
+    if (inFlightRef.current) return;
+    inFlightRef.current = true;
+    fetchState()
+      .then((snapshot) => {
+        setState(snapshot);
+        setError(null);
+        setLoading(false);
+      })
+      .catch((err: unknown) => {
+        setError(err instanceof Error ? err.message : String(err));
+      })
+      .finally(() => {
+        inFlightRef.current = false;
+      });
+  }, []);
+
+  useEffect(() => {
+    refresh(); // fetch immediately on mount, don't wait for the first interval tick
+    const id = setInterval(refresh, intervalMs);
+    return () => clearInterval(id);
+  }, [intervalMs, refresh]);
+
+  return { state, loading, error, refresh };
+}