feat(examples): add saas tutorial skeleton

Add a compile-tested v1 tutorial skeleton that proves the production-shaped app boundary without provider-heavy scope: auth/session context, CSRF-checked mutation, local persistence adapter, generated form/slot/keyed row/page/push effects, plain CSS, SSE shape, and one explicit metrics island.

req: examples/001

req: auth/001

req: auth/002

req: auth/004

req: form/001

req: failure/004

req: page_swap/002

req: push/003

examples/saas/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    hemx_build::app().run().unwrap();
+}