Implement canonical navigation phase 2

[pcharbon70]

Summary

• lower canonical navigation transitions into stable compiler descriptors
• extend UnifiedIUR with maintained navigation descriptor, fixture, export, inspect, and transport helpers
• add phase 2 integration coverage across UnifiedUi, UnifiedIUR, LiveUi, DesktopUi, and TerminalUi

Validation

• cd packages/unified-ui && mix test test/unified_ui/canonical_navigation_integration_test.exs
• cd packages/unified-ui && mix unified_ui.validate
• cd packages/unified_iur && mix test test/unified_iur/interaction_test.exs test/unified_iur/reference_test.exs test/unified_iur/fixtures_test.exs test/unified_iur/inspect_test.exs test/unified_iur/export_test.exs test/unified_iur/interactions_transport_test.exs test/unified_iur/canonical_navigation_boundary_integration_test.exs test/unified_iur/phase_4_integration_test.exs test/unified_iur/phase_6_integration_test.exs test/unified_iur_test.exs
• cd packages/unified_iur && mix unified_iur.validate
• cd packages/live_ui && mix test test/live_ui/canonical_navigation_transport_integration_test.exs
• cd packages/desktop_ui && mix test test/desktop_ui/canonical_navigation_transport_integration_test.exs
• cd packages/terminal_ui && mix test test/terminal_ui/canonical_navigation_transport_integration_test.exs
• mix spec.verify --debug (repo baseline: 51 errors / 236 warnings)
• mix spec.check (repo baseline: 51 errors / 236 warnings)
• mix spec.diffcheck (AGENTS.md remains the only unmapped local diffcheck finding)