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>

sx/sxc/pages/helpers.py

@@ -28,6 +28,9 @@ def _register_sx_helpers() -> None:
         "run-modular-tests": _run_modular_tests,
         "streaming-demo-data": _streaming_demo_data,
         "affinity-demo-data": _affinity_demo_data,
+        "optimistic-demo-data": _optimistic_demo_data,
+        "action:add-demo-item": _add_demo_item,
+        "offline-demo-data": _offline_demo_data,
     })
 
 
@@ -922,3 +925,45 @@ def _affinity_demo_data() -> dict:
             })
 
     return {"components": components, "page-plans": page_plans}
+
+
+def _optimistic_demo_data() -> dict:
+    """Return demo data for the optimistic update test page."""
+    from datetime import datetime, timezone
+
+    return {
+        "items": [
+            {"id": 1, "label": "First item", "status": "confirmed"},
+            {"id": 2, "label": "Second item", "status": "confirmed"},
+            {"id": 3, "label": "Third item", "status": "confirmed"},
+        ],
+        "server-time": datetime.now(timezone.utc).isoformat(timespec="seconds"),
+    }
+
+
+def _add_demo_item(**kwargs) -> dict:
+    """Action: add a demo item. Returns confirmation with new item."""
+    from datetime import datetime, timezone
+    import random
+
+    label = kwargs.get("label", "Untitled")
+    return {
+        "id": random.randint(100, 9999),
+        "label": label,
+        "status": "confirmed",
+        "added-at": datetime.now(timezone.utc).isoformat(timespec="seconds"),
+    }
+
+
+def _offline_demo_data() -> dict:
+    """Return demo data for the offline data layer test page."""
+    from datetime import datetime, timezone
+
+    return {
+        "notes": [
+            {"id": 1, "text": "First note", "created": "2026-03-08T10:00:00Z"},
+            {"id": 2, "text": "Second note", "created": "2026-03-08T11:30:00Z"},
+            {"id": 3, "text": "Third note", "created": "2026-03-08T14:15:00Z"},
+        ],
+        "server-time": datetime.now(timezone.utc).isoformat(timespec="seconds"),
+    }