Adds the plugin-facing UI registration surface (Task 9, final D.2b task).
Plugins call host.Ui.AddMarkupPanel(path, binding) from Enable(); calls are
buffered in BufferedUiRegistry before the GL window opens, then drained into
UiHost.Root in GameWindow.OnLoad inside the RetailUi block after the first-
party vitals panel. Faulty plugin markup is isolated (try/catch per panel,
logged + skipped). IPluginHost.Ui added; AppPluginHost wired; StubHost in
Core.Tests updated; BufferedUiRegistryTests confirms drain-once semantics.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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