stage-30 fix: SW cache strategy — bump version + stale-while-revalidate

User reported mobile nav still looked unresponsive. Verified the new
nav-toggle CSS and markup are deployed; the browser's service worker
was serving the prior cockpit.css cache-first from psyc-v1.

Two fixes so this can't recur:
- CACHE_VERSION bumped to psyc-v2 → the activate handler now wipes
  the v1 cache on the user's next visit.
- Static-asset strategy changed from cache-first to
  stale-while-revalidate: serve from cache for instant render, then
  refresh in the background so the *next* load picks up the new CSS/JS
  with no manual cache-busting. Falls back to network if cold.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/psyc/cockpit/static/sw.js

@@ -5,7 +5,7 @@
 // This makes the cockpit installable as a PWA and survives flaky connections,
 // without serving stale operational data behind the operator's back.
 
-const CACHE_VERSION = "psyc-v1";
+const CACHE_VERSION = "psyc-v2";
 const STATIC_ASSETS = [
   "/static/cockpit.css",
   "/static/psyc-tokens.css",
@@ -39,13 +39,19 @@ self.addEventListener("fetch", (event) => {
   const req = event.request;
   if (req.method !== "GET") return;
   if (isStatic(req)) {
-    // cache-first
+    // stale-while-revalidate: serve from cache for speed, refresh in the
+    // background so subsequent loads pick up new CSS/JS without a manual
+    // version bump. Falls back to network if the cache is cold.
     event.respondWith(
-      caches.match(req).then((hit) => hit || fetch(req).then((resp) => {
-        const copy = resp.clone();
-        caches.open(CACHE_VERSION).then((c) => c.put(req, copy)).catch(() => {});
-        return resp;
-      }))
+      caches.open(CACHE_VERSION).then((cache) =>
+        cache.match(req).then((hit) => {
+          const network = fetch(req).then((resp) => {
+            if (resp && resp.ok) cache.put(req, resp.clone()).catch(() => {});
+            return resp;
+          }).catch(() => hit);
+          return hit || network;
+        })
+      )
     );
   } else {
     // network-first for HTML + API