test: realign nav-wiring integration tests with current BuildNavGroups

[laurentiu021]

Summary

The MainWindowViewModelTests nav-wiring assertions had drifted from the current
MainWindowViewModel.BuildNavGroups() — they described an older sidebar layout. The
integration-test project is compile-checked in CI but not executed, so the stale
assertions went unnoticed. This brings every assertion back in lock-step with the
live graph and adds two data-driven guards so future drift fails automatically.

Drift corrected (verified against BuildNavGroups())

• Total leaf count: 56 → 55.
• System group: 8 → 9 (it includes nav-restore-points).
• Cleanup group: 5 → 4 (nav-file-shredder lives under Privacy & Security, not Cleanup).
• Network: replaced the separate nav-dns-changer + nav-hosts-editor with the
  merged nav-dns-hosts tab.
• Re-bucketed the per-group id assertions to match reality: Monitor 6, Apps 3,
  Privacy & Security 9, Info 5 (now lists nav-system-report), Advanced 2.

Per-group counts re-derived from source: dashboard 1, system 9, gaming 5, monitor 6,
cleanup 4, storage 2, network 5, apps 3, privacy 9, customization 4, info 5,
advanced 2 = 55.

Anti-drift guards added

• NavLeaf_EveryItemHasContentAndResolvableView — enumerates the live NavItems and
  asserts each leaf has a non-empty id, non-null Content, and a ViewType that is a
  real UserControl subclass (so the lazy View getter can instantiate it).
• NavGroups_EveryLeafBelongsToExactlyOneGroup — asserts each flat NavItem is owned
  by exactly one group, catching duplicate/orphan wiring.

These enumerate the real tree rather than hard-coding ids, so a future nav change that
breaks the contract fails the suite instead of being silently re-baselined.

Verification

• SysManager.IntegrationTests builds 0 warnings / 0 errors.
• Assertion numbers cross-checked against BuildNavGroups() (MainWindowViewModel.cs).

test: only — touches no product code, no release.