Freeze right columns

[MMihran]

This MR adds support for freezing columns on the right side.
It used draft MR #973 as a base, with major fixes and improvements:

• Corrected column positioning calculations
• Fixed hover states
• Fixed scroll shadows
• Added handling for column groupings

This addresses the right-side freeze column feature requested in multiple issues: #469, #522, #919, and #957.
The API remains backward compatible, following the proposal here by @jassmith:

interface DataEditorProps {
    // ...
    freezeColumns: number | [left: number, right: number];
}

[Copilot]

Pull Request Overview

This pull request adds support for freezing columns on the right side by extending the freezeColumns API to accept a tuple of [left, right] values and updating numerous grid‐rendering, selection and event-handling routines accordingly. Key changes include:

• Computing and propagating distinct freezeLeftColumns and freezeRightColumns values throughout the grid code.
• Updating rendering functions (headers, cells, grid lines, blitting) to properly account for right-side frozen columns.
• Updating relevant tests and stories to validate the new right-freeze functionality.

Reviewed Changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/source/src/use-collapsing-groups.ts	Updated iteration over columns using freezeLeftColumns and freezeRightColumns for collapsing groups.
packages/core/test/*	Added tests verifying freeze column behavior including both trailing (right-side) and left-side freezing.
packages/core/src/internal/...	Updated grid rendering (scrolling, rings, header, cells, blit, etc.) and related utilities to support the new API.
packages/core/src/data-editor/data-editor.tsx	Modified freezing calculations and rendering of freezeRegions to handle separate left/right freeze areas.
packages/core/src/docs/examples/freeze-columns.stories.tsx	Updated story and controls to demonstrate the new freezeColumns prop structure.

Comments suppressed due to low confidence (3)

packages/core/src/internal/data-grid/render/data-grid-lib.ts:28

• Consider adding an inline comment here explaining that freezeColumnsLeft and freezeColumnsRight are derived from the union type and used to determine which columns should remain sticky at the left and right sides respectively. This will help future maintainers understand the design rationale.

    const freezeColumnsRight = typeof freezeColumns === "number" ? 0 : freezeColumns[1];

packages/core/src/data-editor/data-editor.tsx:2499

• The revised computation of freezeRegions now produces separate regions for the left and right frozen columns. Please ensure that all downstream consumers of freezeRegions (e.g. for event handling or redraw calculations) correctly support multiple freeze regions.

            const freezeLeftRegion =

packages/core/src/internal/data-grid/render/data-grid-render.blit.ts:128

• The updated blitting calculation now subtracts both the left and right sticky widths. Please verify that this logic consistently covers all scrolling scenarios and minimizes redraws especially when either the left or right freeze areas are active.

        // blit X