[codex] implement canonical navigation phase 1

[pcharbon70]

Summary

• implement the Phase 1 canonical navigation action vocabulary and target-kind helpers in UnifiedUi.Signal and UnifiedUi.Signals
• add author-time validation, reference surfaces, and tooling diagnostics for canonical screen transitions and local destinations
• update the maintained themed_signal_workspace example and the bindings/interactions guide to show local navigation, screen transitions, and modal transitions
• add end-to-end canonical navigation integration coverage and mark Phase 1 complete in the planning tracker

Validation

• cd packages/unified-ui && mix test test/unified_ui/canonical_navigation_integration_test.exs test/unified_ui/signals_test.exs test/unified_ui/theme_signal_validation_test.exs test/unified_ui/tooling_test.exs test/unified_ui/validation_test.exs test/unified_ui/mix_tasks_test.exs test/unified_ui/examples_test.exs
• cd packages/unified-ui && mix unified_ui.validate
• mix spec.verify --debug (existing repo baseline: 51 errors, 236 warnings)
• mix spec.check (same existing repo baseline)
• mix spec.diffcheck (existing unmapped AGENTS.md change)