卫戍协议：盟约 web remake — initial import

Online co-op browser remake of Arknights' Stronghold Protocol: Alliance
(client-simulated combat, official waves/bonds/items/strategies, 3D board,
facing wheel, operator loadout, official emotes, deployment scripts).

public/js/render/boardArt.js

@@ -0,0 +1,45 @@
+// render/boardArt.js — optional real board art from the local game client (DESIGN §13).
+//
+//   const art = await loadBoardArt(assets)   // null when the art (or its crop table) is not installed
+//   → { key, tiles, images: { D, common?, BG? } }   (tiles = tiles.json of tools/crop-board-atlas.mjs)
+//
+// Discovery never guesses URLs: the local-art manifest (data/local-assets.json, assets.local()) must list the atlas
+// `map/autochess → TX_autochessi_D`; its crop table `tiles.json` sits next to it (written by
+// tools/crop-board-atlas.mjs). Every image referenced by the table is loaded through the shared image cache; a
+// missing image drops only the materials that use it. Everything is cached per page (one load for every view).
+
+let cached = null;
+
+const isObj = (v) => v !== null && typeof v === 'object' && !Array.isArray(v);
+
+/** @param {any} assets store of public/js/assets.js (needs local(), localUrl(), image()) */
+export function loadBoardArt(assets) {
+  if (cached) return cached;
+  cached = (async () => {
+    if (!assets || typeof assets.local !== 'function' || typeof assets.image !== 'function') return null;
+    const manifest = await assets.local().catch(() => null);
+    if (!manifest) return null;
+    const atlasUrl = assets.localUrl ? assets.localUrl('map/autochess', 'TX_autochessi_D') : null;
+    if (!atlasUrl) return null;
+    const dir = atlasUrl.replace(/\/[^/]*$/, '');
+    let tiles = null;
+    try {
+      const res = await fetch(`${dir}/tiles.json`, { cache: 'no-cache' });
+      tiles = res.ok ? await res.json() : null;
+    } catch { tiles = null; }
+    if (!isObj(tiles) || !isObj(tiles.materials) || !isObj(tiles.source)) return null;
+    const images = {};
+    await Promise.all(Object.entries(tiles.source).map(async ([k, s]) => {
+      const url = isObj(s) && typeof s.path === 'string' ? s.path : null;
+      if (!url) return;
+      const img = await assets.image(url).catch(() => null);
+      if (img && img.width > 0) images[k] = img;
+    }));
+    if (!images.D) return null;
+    return { key: `${atlasUrl}#${tiles.version || 0}`, tiles, images };
+  })().catch(() => null);
+  return cached;
+}
+
+/** Forget the cached art (tests / hot reload). */
+export function resetBoardArt() { cached = null; }