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/test-deps.sx

@@ -263,3 +263,43 @@
 
   (deftest "unknown name targets server"
     (assert-equal "server" (render-target "~nonexistent" (test-env) (list "fetch-data")))))
+
+
+;; --------------------------------------------------------------------------
+;; 7. page-render-plan — per-page boundary plan
+;; --------------------------------------------------------------------------
+
+;; A page component that uses both pure and IO components
+(defcomp ~plan-page (&key data)
+  (div
+    (~dep-auto-pure :x "hello")
+    (~dep-auto-io :x data)
+    (~dep-force-client :x "interactive")))
+
+(defsuite "page-render-plan"
+
+  (deftest "plan classifies components correctly"
+    (let ((plan (page-render-plan "(~plan-page :data d)" (test-env) (list "fetch-data"))))
+      ;; ~plan-page has transitive IO deps (via ~dep-auto-io) so targets server
+      (assert-equal "server" (dict-get (get plan :components) "~plan-page"))
+      (assert-equal "client" (dict-get (get plan :components) "~dep-auto-pure"))
+      (assert-equal "server" (dict-get (get plan :components) "~dep-auto-io"))
+      (assert-equal "client" (dict-get (get plan :components) "~dep-force-client"))))
+
+  (deftest "plan server list contains IO components"
+    (let ((plan (page-render-plan "(~plan-page :data d)" (test-env) (list "fetch-data"))))
+      (assert-true (contains? (get plan :server) "~dep-auto-io"))))
+
+  (deftest "plan client list contains pure components"
+    (let ((plan (page-render-plan "(~plan-page :data d)" (test-env) (list "fetch-data"))))
+      (assert-true (contains? (get plan :client) "~dep-auto-pure"))
+      (assert-true (contains? (get plan :client) "~dep-force-client"))))
+
+  (deftest "plan collects IO deps from server components"
+    (let ((plan (page-render-plan "(~plan-page :data d)" (test-env) (list "fetch-data"))))
+      (assert-true (contains? (get plan :io-deps) "fetch-data"))))
+
+  (deftest "pure-only page has empty server list"
+    (let ((plan (page-render-plan "(~dep-auto-pure :x 1)" (test-env) (list "fetch-data"))))
+      (assert-equal 0 (len (get plan :server)))
+      (assert-true (> (len (get plan :client)) 0)))))