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/pages.py

@@ -884,6 +884,9 @@ def auto_mount_pages(app: Any, service_name: str) -> None:
     # Mount IO proxy endpoint for Phase 5: client-side IO primitives
     mount_io_endpoint(app, service_name)
 
+    # Mount service worker at root scope for offline data layer
+    mount_service_worker(app)
+
 
 def mount_pages(bp: Any, service_name: str,
                 names: set[str] | list[str] | None = None) -> None:
@@ -1186,3 +1189,39 @@ def mount_io_endpoint(app: Any, service_name: str) -> None:
         methods=["GET", "POST"],
     )
     logger.info("Mounted IO proxy for %s: %s", service_name, sorted(_ALLOWED_IO))
+
+
+# ---------------------------------------------------------------------------
+# Service Worker mount
+# ---------------------------------------------------------------------------
+
+_SW_MOUNTED = False
+
+def mount_service_worker(app: Any) -> None:
+    """Mount the SX service worker at /sx-sw.js (root scope).
+
+    The service worker provides offline data caching:
+    - /sx/data/* and /sx/io/* responses cached in IndexedDB
+    - /static/* assets cached via Cache API (stale-while-revalidate)
+    - Everything else passes through to network
+    """
+    global _SW_MOUNTED
+    if _SW_MOUNTED:
+        return
+    _SW_MOUNTED = True
+
+    sw_path = os.path.join(
+        os.path.dirname(__file__), "..", "static", "scripts", "sx-sw.js"
+    )
+    sw_path = os.path.abspath(sw_path)
+
+    async def serve_sw():
+        from quart import send_file
+        return await send_file(
+            sw_path,
+            mimetype="application/javascript",
+            cache_timeout=0,  # no caching — SW updates checked by browser
+        )
+
+    app.add_url_rule("/sx-sw.js", endpoint="sx_service_worker", view_func=serve_sw)
+    logger.info("Mounted service worker at /sx-sw.js")