feat(build): generate page-boundary helper

Expose generated ui::page as the canonical page-boundary rendering helper and move the Workout golden path and docs off beginner-visible ui::render calls.

req: public_api/002

req: codegen/002

req: canonical_authoring/006

docs/versioning.md

@@ -15,8 +15,8 @@ changes:
 - `#[hemx::surface]`, `#[hemx::app]`, `#[hemx::component]`, `#[hemx::handler]`,
   and `#[hemx::form]`
 - generated component helpers for slots, keyed partials, forms, handles, page
-  targets, class tokens, and events
-- `hemx::page(...)` for page-boundary rendering
+  targets, page-boundary rendering, class tokens, and events
+- `hemx::page(...)` only at explicit server shell boundaries
 - tuple `IntoEffect` composition
 - `Result<impl IntoEffect, E>` handlers with `IntoHandlerFailure`
 - generated form commands such as `clear`, `reset`, `error`, and `focus`