CLI templates and package components drift apart #52

Summary

Every component exists in up to three copies: the NuGet package (src/ShellUI.Components), the CLI templates (src/ShellUI.Templates) and the demo app. They have drifted: about 50 components differ between package and template, and the APIs don't always match. Examples:

• Pagination: the template raises OnPageChange, the package CurrentPageChanged.
• Sheet / Drawer: the CLI version needs Compositional="true", the package detects it.

Code written against the docs can then work with one install method and not the other.

Proposal

1. Reconcile the differences component by component, keeping the better of each copy (worst offenders: AppSidebar, Toast, DatePicker, Carousel, HoverCard, ThemeToggle, TimePicker, Select, Popover, Stepper, Pagination, Tabs).
2. Generate the CLI templates from the package components at build time, with rules for the namespace, the JS path (module import vs window.ShellUI) and the ClassName alias.
3. Have the demo install everything through the CLI.
4. Extend TemplateSyncTests to every component, with an explicit allowed-drift list.

This is large; it can be split into one PR per group of components.