feature: deferred routing

[Mastersam07]

Motivation

Deferred shell branches shipped in 0.21.0-dev.1 (KaiselBranch.deferred(loadLibrary: feature.loadLibrary)) — a tab's code loads on first activation, with errorBuilder + retry. But deferral is only available at branch granularity. A heavy route on the main stack (an editor, a report viewer, an admin area) still ships in the initial bundle even if most users never open it. On Flutter web that's paid in initial JS download; on Android it blocks using deferred components per feature.

Proposal

First-class per-route deferral with parity to what branches already have:

import 'package:app/editor/editor.dart' deferred as editor;

builder: (context, route) => switch (route) {
  EditorRoute() => KaiselDeferredScreen(
    load: editor.loadLibrary,
    builder: (context) => editor.EditorScreen(),
    // loadingBuilder / errorBuilder with retry, matching KaiselBranch.deferred
  ),
  ...
},

Design questions

• Deep links & restoration: a URL/restored stack landing directly on a deferred route must trigger the load before first build — the loading state appears inside the route's page, not as a blank app.
• Transitions: pushing a deferred route ideally delays the page transition until the library resolves (or animates into the loading state — decide which default).
• Guards: guards run on values, so they need no code from the deferred library — worth stating as a guarantee.
• loadLibrary caching: Dart already memoizes it; the widget just needs idempotent handling.

Non-goals

Automatic inference of what to defer — the deferred as import stays an explicit app decision, same as branches.