Phase 4: Client-side routing for SX expression URLs

Add sx-url-to-path to router.sx that converts SX expression URLs to
old-style slash paths for route matching. find-matching-route now
transparently handles both formats — the browser URL stays as the SX
expression while matching uses the equivalent old-style path.

/(language.(doc.introduction)) → /language/docs/introduction for matching
but pushState keeps the SX URL in the browser bar.

- router.sx: add _fn-to-segment (doc→docs, etc.), sx-url-to-path
- router.sx: modify find-matching-route to convert SX URLs before matching
- Rebootstrap sx-browser.js and sx_ref.py

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

shared/sx/ref/sx_ref.py

@@ -3973,4 +3973,4 @@ def render(expr, env=None):
 
 def make_env(**kwargs):
     """Create an environment with initial bindings."""
-    return _Env(dict(kwargs))
+    return _Env(dict(kwargs))