Grid aria-colcount

Is your feature request related to a problem? Please describe. The grid container is missing the aria-colcount attribute on its wrapper element. This is important for accessibility, especially for screen readers to announce the total number of columns in the grid.

Currently, we have an updateRowCount() method which sets aria-rowcount. We need a similar mechanism for the column count.

This is also a prerequisite for testing buffered columns.

Describe the solution you'd like

1. Add 'aria-colcount': 0 to the grid's wrapper vdom node in src/grid/Container.mjs.
2. Create a new method updateColCount() which sets the aria-colcount attribute based on the columns.count reactive property and triggers a me.update().
3. The columns collection gets a mutate listener assigned directly within createColumns(). This listener calls onColumnsMutate().
4. A new onColumnsMutate() method calls updateColCount() whenever columns are added or removed from the collection at runtime.
5. Call updateColCount() at the end of construct() to set the initial value.
6. Call updateColCount() inside afterSetColumns() to handle cases where the entire columns config is replaced.