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/techdemo/README.md

@@ -18,7 +18,7 @@ This is a polished Linear-style product demo for planning typed work across lane
 - page-enhancer navigation with native link fallback
 - SSE server push into a generated slot
 - drag-and-drop lane moves persisted by typed server handlers through the slhx runtime
-- an opaque canvas island fed by `Effect::event`/`CustomEvent`, without teaching slhx core about the widget
+- an explicit advanced opaque canvas island fed by a generated event helper, without teaching slhx core about the widget
 - no user-authored browser JavaScript in slhx-managed UI; the island JavaScript is a leaf-widget escape hatch
 
 Verification: