feat: implement double-click auto-fit for dgrid columns

[GordonSmith]

Signed-off-by: Gordon Smith GordonJSmith@gmail.com

WIP

Checklist:

• The commit message is properly formatted and free of typos.
  • The commit message title makes sense in a changelog, by itself.
  • The commit message includes a "fixes" reference if appropriate.
  • The commit is signed.
• The change has been fully tested:
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• There are no similar instances of the same problem that should be addressed
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Testing:

[Copilot]

Pull request overview

This PR implements a double-click auto-fit feature for dgrid table columns, allowing users to automatically adjust column width to fit content by double-clicking the resize handle. The feature is currently marked as Work In Progress (WIP).

Changes:

• Adds double-click detection logic in dgrid-shim to emit custom events when resize handles are double-clicked
• Implements column auto-fitting in the Table component that calculates optimal width based on cell content
• Updates demo/test HTML file to showcase the new feature with multiple table examples

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 9 comments.

File	Description
packages/dgrid-shim/src/gridHelper.ts	Adds mousedown capture listener to detect double-clicks on resize handles and dispatch dgrid-column-autofit custom event; implements resizeColumn helper method to programmatically resize columns
packages/dgrid/src/Table.ts	Adds _autoFitColumn method to calculate optimal column width based on data; registers event listener for dgrid-column-autofit events in update method
packages/dgrid/index.html	Converts from single table demo to multi-table demo showcasing both nested columns and resize handle functionality