docs/feat: sealed routes can't be enumerated at runtime — document the testing impact

This is the honest cost of being codegen-free rather than a bug, but it changes how apps test and is currently undocumented.

With a route *table* (auto_route, go_router) you can walk it at runtime and assert over **every** route: "these N are auth-protected", "these are the analytics names", "no route is unreachable". With sealed types, Dart cannot enumerate a sealed types subtypes at runtime, so those invariants have nowhere typed to iterate.

## What apps end up doing

Both such tests in one migration became regex passes over the route source files:

```dart
final routeFiles = Directory(lib/.../routes).listSync().whereType();
RegExp("String get routeName => ([A-Za-z0-9_]+)")
.allMatches(file.readAsStringSync());
```

That catches drift, but a text search is standing in for a type-level property.

It bites hardest on safety-critical invariants. An "is this screen reachable while locked?" classification is a `switch` over route types with a default. Verifying it exhaustively needs an instance of every route — and routes carrying domain objects cannot be cheaply constructed in a test. In that app only 28 of 41 relevant routes could be instantiated; the rest had to be covered by grepping the classifications own source.

## What already works

Exhaustiveness is fully solved for anything that **dispatches** on a route — page builders, codecs, transition wrappers all get compile-time totality from `switch`. The gap is specifically invariants over the **set** of routes.

## Suggestion

At minimum, document the trade-off and the source-level testing pattern so teams do not discover it when writing their first whole-table test.

Optionally, endorse a convention the compiler can help with:

```dart
sealed class AppRoute extends KaiselRoute {
const AppRoute();
static const all = [Home(), Settings(), /* ... */];
}
```

Hand-maintained, but usable by whole-table tests, and drift is at least visible in review. A lint (`pkg:kaisel_lint`) that flags a route variant missing from such a list would make it reliable.

No documentation file or test entry point is named. Start by locating the project's existing routing documentation and route-testing guidance, then document that sealed routes cannot be enumerated at runtime and describe the source-level testing pattern shown in the issue. Done means the trade-off and its impact on whole-route invariants are clear to app authors; the optional convention should be treated as a separate scope decision.

Written by the indexing model from the issue text.