feat: data table for MAUI and Avalonia

[Shewart]

Summary

Adds data-table (DataTable, DataTableColumn) for MAUI and Avalonia: a Table filled from a list of objects. The registry now has 103 entries (100 CLI targets, 69 families).

• Data: fills from any list and follows an ObservableCollection as it changes. Each column names the item property it shows, with an optional format string.
• Filter: a field above the table; every typed word must match some column.
• Sort: a sortable header cycles ascending, descending and unsorted; numbers and dates sort by value.
• Selection: an optional checkbox column, with a header checkbox that selects every filtered row.
• Pages: Previous / Next with a page size.

Both demos show it with twelve sample invoices, and the docs cover it.

Notes

• Columns read properties by name at run time, which trimming can remove. A Value function set in code is the trim-safe alternative; the docs show it.
• Not ported from the Blazor version: row action menus and server-side paging. Cells show text only.
• It depends on table, input, checkbox and button, which add data-table installs.
• On Avalonia, sortable headers are tab stops and keep focus after a keyboard sort, and a click on a row's checkbox does not raise RowTapped.

Verification

• 984 tests pass; format is clean.
• The MAUI demo (Windows target) and the Avalonia demo build with 0 warnings and 0 errors.
• data-table installs alone into a fresh Avalonia app and a fresh dotnet new maui app (Windows target), building with 0 warnings.
• MAUI demo checked on Windows in dark and light.
• Avalonia demo checked on Windows in dark and light with pointer and key input.