Phase 7c+7d: cache invalidation + offline data layer

7c: Client data cache management via element attributes
(sx-cache-invalidate) and response headers (SX-Cache-Invalidate,
SX-Cache-Update). Programmatic API: invalidate-page-cache,
invalidate-all-page-cache, update-page-cache.

7d: Service Worker (sx-sw.js) with IndexedDB for offline-capable
data caching. Network-first for /sx/data/ and /sx/io/, stale-while-
revalidate for /static/. Cache invalidation propagates from
in-memory cache to SW via postMessage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/bootstrap_js.py

@@ -3551,6 +3551,27 @@ PLATFORM_ORCHESTRATION_JS = """
     });
   }
 
+  function parseSxData(text) {
+    // Parse SX text into a data value. Returns the first parsed expression,
+    // or NIL on error. Used by cache update directives.
+    try {
+      var exprs = parse(text);
+      return exprs.length >= 1 ? exprs[0] : NIL;
+    } catch (e) {
+      logWarn("sx:cache parse error: " + (e && e.message ? e.message : e));
+      return NIL;
+    }
+  }
+
+  function swPostMessage(msg) {
+    // Send a message to the active service worker (if registered).
+    // Used to notify SW of cache invalidation.
+    if (typeof navigator !== "undefined" && navigator.serviceWorker &&
+        navigator.serviceWorker.controller) {
+      navigator.serviceWorker.controller.postMessage(msg);
+    }
+  }
+
   function urlPathname(href) {
     try {
       return new URL(href, location.href).pathname;
@@ -4025,6 +4046,14 @@ def public_api_js(has_html, has_sx, has_dom, has_engine, has_orch, has_boot, has
       }
       // Set up direct resolution for future chunks
       global.__sxResolve = function(id, sx) { resolveSuspense(id, sx); };
+      // Register service worker for offline data caching
+      if ("serviceWorker" in navigator) {
+        navigator.serviceWorker.register("/sx-sw.js", { scope: "/" }).then(function(reg) {
+          logInfo("sx:sw registered (scope: " + reg.scope + ")");
+        }).catch(function(err) {
+          logWarn("sx:sw registration failed: " + (err && err.message ? err.message : err));
+        });
+      }
     };
     if (document.readyState === "loading") {
       document.addEventListener("DOMContentLoaded", _sxInit);