Phase 4: Client-side rendering of :data pages via abstract resolve-page-data

Spec layer (orchestration.sx):
- try-client-route now handles :data pages instead of falling back to server
- New abstract primitive resolve-page-data(name, params, callback) — platform
  decides transport (HTTP, IPC, cache, etc)
- Extracted swap-rendered-content and resolve-route-target helpers

Platform layer (bootstrap_js.py):
- resolvePageData() browser implementation: fetches /sx/data/<name>, parses
  SX response, calls callback. Other hosts provide their own transport.

Server layer (pages.py):
- evaluate_page_data() evaluates :data expr, serializes result as SX
- auto_mount_page_data() mounts /sx/data/ endpoint with per-page auth
- _build_pages_sx now computes component deps for all pages (not just pure)

Test page at /isomorphism/data-test exercises the full pipeline.

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

sx/sx/data-test.sx

@@ -0,0 +1,52 @@
+;; Data test page — exercises Phase 4 client-side data rendering.
+;;
+;; This page has a :data expression. When navigated to:
+;; - Full page load: server evaluates data + renders content (normal path)
+;; - Client route: client fetches /sx/data/data-test, parses SX, renders locally
+;;
+;; Open browser console and look for "sx:route client+data" to confirm
+;; client-side rendering happened.
+
+(defcomp ~data-test-content (&key server-time items phase transport)
+  (div :class "space-y-8"
+    (div :class "border-b border-stone-200 pb-6"
+      (h1 :class "text-2xl font-bold text-stone-900" "Data Test")
+      (p :class "mt-2 text-stone-600"
+        "This page tests the Phase 4 data endpoint. The content you see was "
+        "rendered using data from the server, but the rendering itself may have "
+        "happened client-side."))
+
+    ;; Server-provided metadata
+    (div :class "rounded-lg border border-stone-200 bg-white p-6 space-y-3"
+      (h2 :class "text-lg font-semibold text-stone-800" "Data from server")
+      (dl :class "grid grid-cols-2 gap-2 text-sm"
+        (dt :class "font-medium text-stone-600" "Phase")
+        (dd :class "text-stone-900" phase)
+        (dt :class "font-medium text-stone-600" "Transport")
+        (dd :class "text-stone-900" transport)
+        (dt :class "font-medium text-stone-600" "Server time")
+        (dd :class "font-mono text-stone-900" server-time)))
+
+    ;; Pipeline steps from data
+    (div :class "space-y-3"
+      (h2 :class "text-lg font-semibold text-stone-800" "Pipeline steps")
+      (div :class "space-y-2"
+        (map-indexed
+          (fn (i item)
+            (div :class "flex items-start gap-3 rounded border border-stone-100 bg-white p-3"
+              (span :class "flex-none rounded-full bg-violet-100 text-violet-700 w-6 h-6 flex items-center justify-center text-xs font-bold"
+                (str (+ i 1)))
+              (div
+                (div :class "font-medium text-stone-900" (get item "label"))
+                (div :class "text-sm text-stone-500" (get item "detail")))))
+          items)))
+
+    ;; How to verify
+    (div :class "rounded-lg border border-amber-200 bg-amber-50 p-4 text-sm space-y-2"
+      (p :class "font-semibold text-amber-800" "How to verify client-side rendering")
+      (ol :class "list-decimal list-inside text-amber-700 space-y-1"
+        (li "Open the browser console (F12)")
+        (li "Navigate to this page from another page using a link")
+        (li "Look for: " (code :class "bg-amber-100 px-1 rounded" "sx:route client+data /isomorphism/data-test"))
+        (li "That log line means the data was fetched and rendered client-side")
+        (li "A full page reload will show server-side rendering instead")))))