Phase 4 complete: client data cache + plan update

- Add page data cache in orchestration.sx (30s TTL, keyed by page-name+params)
- Cache hit path: sx:route client+cache (instant render, no fetch)
- Cache miss path: sx:route client+data (fetch, cache, render)
- Fix HTMX response dep computation to include :data pages
- Update isomorphic-sx-plan.md: Phases 1-4 marked done with details,
  reorder remaining phases (continuations→Phase 5, suspense→Phase 6,
  optimistic updates→Phase 7)

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

shared/sx/pages.py

@@ -282,7 +282,7 @@ async def execute_page(
         # Compute content expression deps so the server sends component
         # definitions the client needs for future client-side routing
         extra_deps: set[str] | None = None
-        if page_def.content_expr is not None and page_def.data_expr is None:
+        if page_def.content_expr is not None:
             from .deps import components_needed
             from .parser import serialize
             try: