Implement canonical widget component runtime parity

[pcharbon70]

Summary

• Adds shared runtime parity fixtures for the expanded canonical widget component catalog.
• Implements component parity across ElmUi, DesktopUi, and TerminalUi, including TerminalUi degradation summaries and desktop render-plan support.
• Adds a unified runtime parity integration gate that runs the ElmUi, DesktopUi, and TerminalUi component suites.

Section Commits

• 4.1 d8154761 Add shared widget runtime parity fixtures
• 4.2 ad654c94 Implement ElmUi widget component parity
• 4.3 7675fb7a Implement DesktopUi widget component parity
• 4.4 6ef2e633 Implement TerminalUi widget component parity
• 4.5 f9f70391 Add widget component runtime parity integration gate

Verification

• cd packages/desktop_ui && mix test
• cd packages/terminal_ui && mix test
• cd packages/unified_iur && mix test test/unified_iur/widget_component_runtime_parity_test.exs
• mix spec.check
• mix spec.diffcheck
• git diff --check

Note: a full root mix test run still reports existing live spec-compliance failures across multiple packages; the focused Section 4.5 runtime parity gate passes.