Extend defhandler with :path/:method/:csrf, migrate 12 ref endpoints to SX

defhandler now supports keyword options for public route registration:
  (defhandler name :path "/..." :method :post :csrf false (&key) body)

Infrastructure: forms.sx parses options, HandlerDef stores path/method/csrf,
register_route_handlers() mounts path-based handlers as app routes.

New IO primitives (boundary.sx "Web interop" section): now, sleep,
request-form, request-json, request-header, request-content-type.

First migration: 12 reference API endpoints from Python f-string SX
to declarative .sx handlers in sx/sx/handlers/ref-api.sx.

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

sx/app.py

@@ -103,6 +103,14 @@ def create_app() -> "Quart":
     bp = register_pages(url_prefix="/")
     app.register_blueprint(bp)
 
+    # Register SX-defined route handlers (defhandler with :path)
+    from shared.sx.handlers import register_route_handlers
+    n_routes = register_route_handlers(app, "sx")
+    if n_routes:
+        import logging
+        logging.getLogger("sx.handlers").info(
+            "Registered %d route handler(s) for sx", n_routes)
+
     from shared.sx.pages import auto_mount_pages
     auto_mount_pages(app, "sx")