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>

sx/sx/specs.sx

@@ -182,7 +182,8 @@ continuations.sx    depends on: eval (optional)
 callcc.sx           depends on: eval (optional)
 
 ;; Spec modules (optional — loaded via --spec-modules)
-deps.sx             depends on: eval (optional)")))
+deps.sx             depends on: eval (optional)
+router.sx           (standalone — pure string/list ops)")))
 
       (div :class "space-y-3"
         (h2 :class "text-2xl font-semibold text-stone-800" "Extensions")