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/sx/nav-data.sx

@@ -135,7 +135,9 @@
   (dict :label "Data Test" :href "/isomorphism/data-test")
   (dict :label "Async IO" :href "/isomorphism/async-io")
   (dict :label "Streaming" :href "/isomorphism/streaming")
-  (dict :label "Affinity" :href "/isomorphism/affinity")))
+  (dict :label "Affinity" :href "/isomorphism/affinity")
+  (dict :label "Optimistic" :href "/isomorphism/optimistic")
+  (dict :label "Offline" :href "/isomorphism/offline")))
 
 (define plans-nav-items (list
   (dict :label "Status" :href "/plans/status"