Phase 7b: page render plans — per-page boundary optimizer

Add page-render-plan to deps.sx: given page source + env + IO names,
computes a dict mapping each needed component to "server" or "client",
with server/client lists and IO dep collection. 5 new spec tests.

Integration:
- PageDef.render_plan field caches the plan at registration
- compute_page_render_plans() called from auto_mount_pages()
- Client page registry includes :render-plan per page
- Affinity demo page shows per-page render plans

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

shared/sx/pages.py

@@ -840,6 +840,25 @@ async def execute_page_streaming_oob(
 # Blueprint mounting
 # ---------------------------------------------------------------------------
 
+def compute_page_render_plans(service_name: str) -> None:
+    """Pre-compute and cache render plans for all pages in a service.
+
+    Must be called after components are loaded (compute_all_deps/io_refs done)
+    and pages are registered. Stores plans on PageDef.render_plan.
+    """
+    from .parser import serialize
+    from .deps import page_render_plan, get_all_io_names
+    from .jinja_bridge import _COMPONENT_ENV
+
+    io_names = get_all_io_names()
+    pages = get_all_pages(service_name)
+    for page_def in pages.values():
+        if page_def.content_expr is not None:
+            content_src = serialize(page_def.content_expr)
+            page_def.render_plan = page_render_plan(content_src, _COMPONENT_ENV, io_names)
+    logger.info("Computed render plans for %d pages in %s", len(pages), service_name)
+
+
 def auto_mount_pages(app: Any, service_name: str) -> None:
     """Auto-mount all registered defpages for a service directly on the app.
 
@@ -849,6 +868,10 @@ def auto_mount_pages(app: Any, service_name: str) -> None:
     Also mounts the /sx/data/ endpoint for client-side data fetching.
     """
     pages = get_all_pages(service_name)
+
+    # Pre-compute render plans (which components render where)
+    compute_page_render_plans(service_name)
+
     for page_def in pages.values():
         _mount_one_page(app, service_name, page_def)
     logger.info("Auto-mounted %d defpages for %s", len(pages), service_name)