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>

sx/sxc/pages/helpers.py

@@ -881,9 +881,10 @@ async def _streaming_demo_data():
 
 
 def _affinity_demo_data() -> dict:
-    """Return affinity analysis for the demo components."""
+    """Return affinity analysis for the demo components + page render plans."""
     from shared.sx.jinja_bridge import get_component_env
     from shared.sx.types import Component
+    from shared.sx.pages import get_all_pages
 
     env = get_component_env()
     demo_names = [
@@ -904,4 +905,20 @@ def _affinity_demo_data() -> dict:
                 "io-refs": sorted(val.io_refs),
                 "is-pure": val.is_pure,
             })
-    return {"components": components}
+
+    # Collect render plans from all sx service pages
+    page_plans = []
+    for page_def in get_all_pages("sx").values():
+        plan = page_def.render_plan
+        if plan:
+            page_plans.append({
+                "name": page_def.name,
+                "path": page_def.path,
+                "server-count": len(plan.get("server", [])),
+                "client-count": len(plan.get("client", [])),
+                "server": plan.get("server", []),
+                "client": plan.get("client", []),
+                "io-deps": plan.get("io-deps", []),
+            })
+
+    return {"components": components, "page-plans": page_plans}