feat(axum): add concise interaction request dispatch

Wrap extracted interaction forms in an InteractionRequest with a concise dispatch path, add a handlers() helper, and move canonical examples off direct HandlerRegistry/InteractionForm plumbing.

req: axum_integration/003

req: ceremony/001

req: dx/003

req: examples/003

slhx-test/tests/examples_contract.rs

@@ -63,6 +63,8 @@ fn canonical_examples_do_not_author_low_level_resource_plumbing() {
         "::lower(include_str!",
         "lower_html(",
         "render_html(",
+        "HandlerRegistry",
+        "InteractionForm",
         "name=\"__h\"",
         "name='__h'",
     ];