get_route: label per-branch navigators with branch index

[devoncarew]

From usability study feedback (session 2026-04-16):

When using StatefulShellRoute, get_route output shows multiple per-branch navigators all labelled "navigator":

Navigator: navigator
Route stack (1 entry):
  [1/1] InputsScreen ...
Navigator: navigator
Route stack (1 entry):
  [1/1] FormScreen ...

With 4+ tabs this is ambiguous — you can't tell which branch is which without cross-referencing with a screenshot.

Suggestion: include the branch index in the label (Navigator: branch 0 (Inputs)) when the navigator is a branch navigator under a StatefulShellRoute. If a navigatorKey with a debug label is set, prefer that.