feat(axum): add interaction registry alias

Add an interactions()/on() registration path and move canonical examples off explicit register_handle naming while keeping low-level registry types available underneath.

req: axum_integration/003

req: ceremony/001

req: dx/003

req: examples/003

slhx-test/tests/examples_contract.rs

@@ -65,6 +65,7 @@ fn canonical_examples_do_not_author_low_level_resource_plumbing() {
         "render_html(",
         "HandlerRegistry",
         "InteractionForm",
+        "register_handle(",
         "name=\"__h\"",
         "name='__h'",
     ];