refactor: decompose the KaiselRouterDelegate monolith

@Mastersam07 is already working on this.

Since Jul 20, 2026.

[authored by Randal, assisted by Gemini]

## Problem
`KaiselRouterDelegate` in `packages/kaisel/lib/src/kaisel_router_delegate.dart` is currently a monolithic class spanning over 1,100 lines:

https://github.com/Mastersam07/kaisel/blob/dev/packages/kaisel/lib/src/kaisel_router_delegate.dart

It has multiple responsibilities mixed into a single class:
- Core widget subtree rendering (`Navigator` and `modalBuilder`).
- Deep-linking & State Restoration (`restorationScopeId`).
- System-back and Android predictive back navigation handling.
- Web browser transition configurations.
- DevTools Inspector registrations and hook tracking.
- Nested routing registrations (`KaiselNestedHost`).

This large footprint makes the class difficult to test in isolation, maintain, and audit as the codebase scales.

## Proposed Solution
Refactor and modularize `KaiselRouterDelegate` by delegating single responsibilities to separate helper classes, mixins, or extensions:

- **State Restoration**: Pull out state restoration logic into a dedicated helper class (e.g., `KaiselRestorationHelper`) or mixin.
- **Back Navigation Handling**: Move predictive back and pop handling to a separate class/mixin.
- **Inspector / DevTools Tracking**: Decouple the DevTools registry hooks.
- **Nested Host Registry**: Move tracking of active inner navigators/branches out of the main delegate.

By separating these concerns, the core delegate can focus primarily on listening to the router and rendering the widget tree.