Implement canonical navigation modal stack phases

[pcharbon70]

Summary

• Implemented the canonical navigation action and modal stack backbone across UnifiedUi, UnifiedIUR, shared transport, and runtime packages.
• Added runtime modal stack handling and focused review examples for LiveUi, ElmUi, DesktopUi, and TerminalUi.
• Added integration coverage for shared modal stack fixtures, topmost and targeted close semantics, and host-specific navigation leakage rejection.
• Updated canonical navigation phase checklists and package traceability mirrors.

Verification

• mix test test/unified_iur/canonical_navigation_boundary_integration_test.exs test/unified_iur/interactions_transport_test.exs
• mix test test/live_ui/canonical_navigation_transport_integration_test.exs test/live_ui/canonical_navigation_web_runtime_integration_test.exs test/live_ui/runtime_navigation_test.exs test/live_ui/transport_diagnostics_test.exs
• mix test test/elm_ui/canonical_navigation_web_runtime_integration_test.exs test/elm_ui/runtime_navigation_test.exs test/elm_ui/transport_diagnostics_test.exs
• mix test test/desktop_ui/canonical_navigation_transport_integration_test.exs test/desktop_ui/transport_test.exs test/desktop_ui/navigation/signal_test.exs test/desktop_ui/examples_test.exs
• mix test test/terminal_ui/canonical_navigation_transport_integration_test.exs test/terminal_ui/transport_diagnostics_test.exs test/terminal_ui/transport_signal_test.exs test/terminal_ui/examples_test.exs
• mix spec.plancheck live_ui
• mix spec.plancheck elm_ui
• mix spec.plancheck desktop_ui
• mix spec.plancheck terminal_ui
• mix spec.verify --debug
• mix spec.check
• mix spec.diffcheck