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/ref/bootstrap_js.py

@@ -549,6 +549,7 @@ class JSEmitter:
             "compute-all-io-refs": "computeAllIoRefs",
             "component-pure?": "componentPure_p",
             "render-target": "renderTarget",
+            "page-render-plan": "pageRenderPlan",
             # router.sx
             "split-path-segments": "splitPathSegments",
             "make-route-segment": "makeRouteSegment",