feat(api): streamline generated app authoring

Move the canonical examples toward generated component-root helpers, typed form decoding, async/state handler registration, and derive-driven app/component registry wiring. Tighten requirements and diagnostics for the server-first, selectorless authoring path.

Verified with cargo run -p slhx-xtask -- test, cargo check --workspace, redgate list, redgate refs, redgate health --strict, and git diff --check.

req: canonical/001

req: canonical/003

req: canonical/004

req: dx/002

req: derive_app/001

req: component/003

req: form/004

req: axum_integration/003

examples/v0/templates/page_swap.heml

@@ -1,6 +1,6 @@
 <main data-slhx-root="docs">
   <nav data-slhx-slot="nav">
-    <a href="/docs" data-slhx-nav="" data-slhx-handle="load_docs">Docs</a>
+    <a href="/docs" data-slhx-nav="">Docs</a>
   </nav>
   <article data-slhx-slot="content">{+= self.content =+}</article>
   <title data-slhx-slot="title">{+ self.title +}</title>