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/partials/todo_items.heml

@@ -1,6 +1,3 @@
-<template h-if="self.items.is_empty()">
-  <li>No todos yet</li>
-</template>
-<template h-else>
+<template h-if="!self.items.is_empty()">
   <li h-for="todo in &self.items" +data-key="todo.id">{+ todo.title +}</li>
 </template>