Implement canonical navigation phase 4 non-web runtime mapping

[pcharbon70]

Summary

• align desktop_ui canonical navigation transitions with the existing screen registry, controller, history, and modal model
• add terminal_ui canonical navigation realization for screen replacement, bounded history, and capability-aware modal degradation
• add maintained non-web navigation review artifacts and phase integration coverage for the shared canonical fixture set

Section Commits

• 5d6da541 Implement canonical navigation phase 4 section 4.1
• d28df1c2 Implement canonical navigation phase 4 section 4.2
• 7e76f715 Implement canonical navigation phase 4 section 4.3
• c81b6d34 Implement canonical navigation phase 4 section 4.4

Validation

• cd packages/desktop_ui && mix test test/desktop_ui/navigation/signal_test.exs test/desktop_ui/widgets/navigation_test.exs test/desktop_ui/navigation/integration_test.exs test/desktop_ui/transport_test.exs test/desktop_ui/canonical_navigation_transport_integration_test.exs test/desktop_ui/canonical_navigation_desktop_runtime_integration_test.exs test/desktop_ui/runtime_test.exs
• cd packages/terminal_ui && mix test test/terminal_ui/runtime_test.exs test/terminal_ui/runtime_event_router_test.exs test/terminal_ui/transport_diagnostics_test.exs test/terminal_ui/transport_signal_test.exs test/terminal_ui/foundational_widget_families_test.exs test/terminal_ui/canonical_navigation_transport_integration_test.exs test/terminal_ui/canonical_navigation_terminal_runtime_integration_test.exs
• cd packages/desktop_ui && mix test test/desktop_ui/examples_test.exs
• cd packages/terminal_ui && mix test test/terminal_ui/examples_test.exs test/terminal_ui_test.exs
• cd packages/desktop_ui && mix test test/desktop_ui/canonical_navigation_transport_integration_test.exs test/desktop_ui/canonical_navigation_desktop_runtime_integration_test.exs test/desktop_ui/canonical_navigation_non_web_examples_integration_test.exs
• cd packages/terminal_ui && mix test test/terminal_ui/canonical_navigation_transport_integration_test.exs test/terminal_ui/canonical_navigation_terminal_runtime_integration_test.exs test/terminal_ui/canonical_navigation_non_web_examples_integration_test.exs
• mix spec.verify --debug -> unchanged repo baseline: 51 errors / 236 warnings
• mix spec.check -> unchanged repo baseline: 51 errors / 236 warnings
• mix spec.diffcheck -> unchanged single finding for unmapped AGENTS.md

Notes

• existing desktop_ui warnings about missing LiveUi compile-time modules are unchanged
• unrelated dirty worktree changes were preserved and are not part of this PR

[pcharbon70]

Closing this draft because current main has removed the live_ui demo surface that this branch still modifies. If phase 04 work resumes, it should be rebased onto current main first.