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_row.heml

@@ -1 +1,10 @@
-<li>{+ self.title +}</li>
+<li>
+  <span>{+ self.title +}</span>
+  <form data-slhx-handle="rename_todo_row">
+    <input type="hidden" name="id" +value="self.id">
+    <button type="submit" name="title" value="Renamed todo">Rename</button>
+  </form>
+  <form data-slhx-handle="delete_todo_row">
+    <button type="submit" name="id" +value="self.id">Delete</button>
+  </form>
+</li>