Phase 3: Client-side routing with SX page registry + routing analyzer demo

Add client-side route matching so pure pages (no IO deps) can render
instantly without a server roundtrip. Page metadata serialized as SX
dict literals (not JSON) in <script type="text/sx-pages"> blocks.

- New router.sx spec: route pattern parsing and matching (6 pure functions)
- boot.sx: process page registry using SX parser at startup
- orchestration.sx: intercept boost links for client routing with
  try-first/fallback — client attempts local eval, falls back to server
- helpers.py: _build_pages_sx() serializes defpage metadata as SX
- Routing analyzer demo page showing per-page client/server classification
- 32 tests for Phase 2 IO detection (scan_io_refs, transitive_io_refs,
  compute_all_io_refs, component_pure?) + fallback/ref parity
- 37 tests for Phase 3 router functions + page registry serialization
- Fix bootstrap_py.py _emit_let cell variable initialization bug
- Fix missing primitive aliases (split, length, merge) in bootstrap_py.py

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

shared/sx/ref/boot.sx

@@ -295,6 +295,33 @@
         scripts))))
 
 
+;; --------------------------------------------------------------------------
+;; Page registry for client-side routing
+;; --------------------------------------------------------------------------
+
+(define _page-routes (list))
+
+(define process-page-scripts
+  (fn ()
+    ;; Process <script type="text/sx-pages"> tags.
+    ;; Parses SX page registry and builds route entries with parsed patterns.
+    (let ((scripts (query-page-scripts)))
+      (for-each
+        (fn (s)
+          (when (not (is-processed? s "pages"))
+            (mark-processed! s "pages")
+            (let ((text (dom-text-content s)))
+              (when (and text (not (empty? (trim text))))
+                (let ((pages (parse text)))
+                  (for-each
+                    (fn (page)
+                      (append! _page-routes
+                        (merge page
+                          {"parsed" (parse-route-pattern (get page "path"))})))
+                    pages))))))
+        scripts))))
+
+
 ;; --------------------------------------------------------------------------
 ;; Full boot sequence
 ;; --------------------------------------------------------------------------
@@ -305,12 +332,14 @@
     ;; 1. CSS tracking
     ;; 2. Style dictionary
     ;; 3. Process scripts (components + mounts)
-    ;; 4. Hydrate [data-sx] elements
-    ;; 5. Process engine elements
+    ;; 4. Process page registry (client-side routing)
+    ;; 5. Hydrate [data-sx] elements
+    ;; 6. Process engine elements
     (do
       (init-css-tracking)
       (init-style-dict)
       (process-sx-scripts nil)
+      (process-page-scripts)
       (sx-hydrate-elements nil)
       (process-elements nil))))
 
@@ -354,6 +383,7 @@
 ;; === Script queries ===
 ;;   (query-sx-scripts root)       → list of <script type="text/sx"> elements
 ;;   (query-style-scripts)         → list of <script type="text/sx-styles"> elements
+;;   (query-page-scripts)          → list of <script type="text/sx-pages"> elements
 ;;
 ;; === localStorage ===
 ;;   (local-storage-get key)       → string or nil