docs: refactor fallback design to use RouterAdapter interface

Replace scattered conditional checks with an interface-based approach:

- Define RouterAdapter interface with getSnapshot, subscribe, navigate,
  and setupInterception methods
- NavigationAPIAdapter: wraps real Navigation API
- StaticAdapter: provides read-only fallback behavior
- createAdapter factory selects appropriate implementation

This keeps the Router component and hooks clean, with all mode-specific
logic encapsulated in the adapter implementations.

Author: claude