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/deps.sx

@@ -341,6 +341,50 @@
               :else "client")))))))
 
 
+;; --------------------------------------------------------------------------
+;; 6. Page render plan — pre-computed boundary decisions for a page
+;; --------------------------------------------------------------------------
+;; Given page source + env + IO names, returns a render plan dict:
+;;
+;;   {:components {~name "server"|"client" ...}
+;;    :server     (list of ~names that render server-side)
+;;    :client     (list of ~names that render client-side)
+;;    :io-deps    (list of IO primitives needed by server components)}
+;;
+;; This is computed once at page registration and cached on the page def.
+;; The async evaluator and client router both use it to make decisions
+;; without recomputing at every request.
+
+(define page-render-plan
+  (fn (page-source env io-names)
+    (let ((needed (components-needed page-source env))
+          (comp-targets (dict))
+          (server-list (list))
+          (client-list (list))
+          (io-deps (list)))
+
+      (for-each
+        (fn (name)
+          (let ((target (render-target name env io-names)))
+            (dict-set! comp-targets name target)
+            (if (= target "server")
+              (do
+                (append! server-list name)
+                ;; Collect IO deps from server components
+                (for-each
+                  (fn (io-ref)
+                    (when (not (contains? io-deps io-ref))
+                      (append! io-deps io-ref)))
+                  (transitive-io-refs name env io-names)))
+              (append! client-list name))))
+        needed)
+
+      {:components comp-targets
+       :server server-list
+       :client client-list
+       :io-deps io-deps})))
+
+
 ;; --------------------------------------------------------------------------
 ;; Host obligation: selective expansion in async partial evaluation
 ;; --------------------------------------------------------------------------