Spec URL evaluation in router.sx, bootstrap to Python/JS

Add url-to-expr, auto-quote-unknowns, prepare-url-expr to router.sx —
the canonical URL-to-expression pipeline. Dots→spaces, parse, then
auto-quote unknown symbols as strings (slugs). The same spec serves
both server (Python) and client (JS) route handling.

- router.sx: three new pure functions for URL evaluation
- bootstrap_py.py: auto-include router module with html adapter
- platform_js.py: export urlToExpr/autoQuoteUnknowns/prepareUrlExpr
- sx_router.py: replace hand-written auto_quote_slugs with bootstrapped
  prepare_url_expr — delete ~50 lines of hardcoded function name sets
- Rebootstrap sx_ref.py (4331 lines) and sx-browser.js

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

shared/sx/ref/platform_js.py

@@ -3129,6 +3129,9 @@ def public_api_js(has_html, has_sx, has_dom, has_engine, has_orch, has_boot, has
         api_lines.append('    parseRoutePattern: parseRoutePattern,')
         api_lines.append('    matchRoute: matchRoute,')
         api_lines.append('    findMatchingRoute: findMatchingRoute,')
+        api_lines.append('    urlToExpr: urlToExpr,')
+        api_lines.append('    autoQuoteUnknowns: autoQuoteUnknowns,')
+        api_lines.append('    prepareUrlExpr: prepareUrlExpr,')
 
     if has_dom:
         api_lines.append('    registerIo: typeof registerIoPrimitive === "function" ? registerIoPrimitive : null,')