Phase 7c + 7d: optimistic data updates and offline mutation queue

7c — Optimistic Data Updates:
- orchestration.sx: optimistic-cache-update/revert/confirm + submit-mutation
- pages.py: mount_action_endpoint at /sx/action/<name> for client mutations
- optimistic-demo.sx: live demo with todo list, pending/confirmed/reverted states
- helpers.py: demo data + add-demo-item action handler

7d — Offline Data Layer:
- orchestration.sx: connectivity tracking, offline-queue-mutation, offline-sync,
  offline-aware-mutation (routes online→submit, offline→queue)
- offline-demo.sx: live demo with notes, connectivity indicator, sync timeline
- helpers.py: offline demo data

Also updates plans.sx: marks Phase 7 fully complete (all 6 sub-phases 7a-7f).

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 action endpoint for Phase 7c: optimistic data mutations
+    mount_action_endpoint(app, service_name)
+
 
 def mount_pages(bp: Any, service_name: str,
                 names: set[str] | list[str] | None = None) -> None:
@@ -1186,3 +1189,56 @@ 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))
+
+
+def mount_action_endpoint(app: Any, service_name: str) -> None:
+    """Mount /sx/action/<name> endpoint for client-side data mutations.
+
+    The client can POST to trigger a named action (registered via
+    register_page_helpers with an 'action:' prefix). The action receives
+    the JSON payload, performs the mutation, and returns the new page data
+    as SX wire format.
+
+    This is the server counterpart to submit-mutation in orchestration.sx.
+    """
+    from quart import make_response, request, abort as quart_abort
+    from .parser import serialize
+    from shared.browser.app.csrf import csrf_exempt
+
+    @csrf_exempt
+    async def action_handler(name: str) -> Any:
+        # Look up action helper
+        helpers = get_page_helpers(service_name)
+        action_fn = helpers.get(f"action:{name}")
+        if action_fn is None:
+            quart_abort(404)
+
+        # Parse JSON payload
+        import asyncio as _asyncio
+        data = await request.get_json(silent=True) or {}
+
+        try:
+            result = action_fn(**data)
+            if _asyncio.iscoroutine(result):
+                result = await result
+        except Exception as e:
+            logger.warning("Action %s failed: %s", name, e)
+            resp = await make_response(f'(dict "error" "{e}")', 500)
+            resp.content_type = "text/sx; charset=utf-8"
+            return resp
+
+        result_sx = serialize(result) if result is not None else "nil"
+        resp = await make_response(result_sx, 200)
+        resp.content_type = "text/sx; charset=utf-8"
+        return resp
+
+    action_handler.__name__ = "sx_action"
+    action_handler.__qualname__ = "sx_action"
+
+    app.add_url_rule(
+        "/sx/action/<name>",
+        endpoint="sx_action",
+        view_func=action_handler,
+        methods=["POST"],
+    )
+    logger.info("Mounted action endpoint for %s at /sx/action/<name>", service_name)