feat(DataGrid): add pinned (sticky/frozen) column support

[Copilot]

Pull Request

📖 Description

Implements column pinning for FluentDataGrid (#4652). Columns can be frozen to the left or right edge of the grid and remain visible during horizontal scrolling.

API surface

New Pin parameter on ColumnBase<TGridItem> (applies to PropertyColumn, TemplateColumn, SelectColumn):

<div style="overflow-x: auto;">
    <FluentDataGrid Items="@employees" Style="min-width: max-content;">
        <PropertyColumn Title="ID"      Property="@(e => e.Id)"    Width="60px"  Pin="DataGridColumnPin.Left" />
        <PropertyColumn Title="Name"    Property="@(e => e.Name)"  Width="160px" Pin="DataGridColumnPin.Left" />
        <PropertyColumn Title="Dept"    Property="@(e => e.Dept)" />
        <TemplateColumn Title="Actions" Width="120px" Pin="DataGridColumnPin.Right">
            ...
        </TemplateColumn>
    </FluentDataGrid>
</div>

New enum: DataGridColumnPin — None (default) / Left / Right.

Validation (throws ArgumentException)

• Pinned columns must declare an explicit pixel Width (e.g. "150px"). fr, %, and other units are rejected.
• Left-pinned columns must be contiguous at the start of the column list — each must immediately follow another left-pinned column or be the very first column.
• Right-pinned columns must be contiguous at the end of the column list — each must immediately precede another right-pinned column or be the very last column.

This prevents "floating" pinned columns in the middle of the grid.

Internal changes

File	Change
ColumnBase.razor.cs	Pin public parameter; PinOffsetPx internal property
FluentDataGrid.razor.cs	ValidateAndComputePinnedColumns() + ValidatePinnedColumnConstraints() called from FinishCollectingColumns(); ParsePixelWidth() helper
FluentDataGridCell.razor.cs	Emits col-pinned-left/col-pinned-right CSS classes; inline position: sticky, left/right offset, z-index: 1 (data cells)
FluentDataGrid.razor.css	Background (--fluent-data-grid-pinned-background), separator borders, z-index floors for header/sticky-header intersections
FluentDataGrid.razor.ts	UpdatePinnedColumnOffsets() recalculates live pixel offsets from rendered header widths; called on Initialize() and every resize pointer-move

🎫 Issues

Closes #4652

👩‍💻 Reviewer Notes

• Focus on ValidatePinnedColumnConstraints() in FluentDataGrid.razor.cs — this is where the ordering rules are enforced.
• UpdatePinnedColumnOffsets() in FluentDataGrid.razor.ts uses offsetWidth in Grid mode and clientWidth in Table mode — consistent with how the existing resize logic elsewhere in that file measures columns.
• The --fluent-data-grid-pinned-background CSS custom property can be set per-grid to theme the pinned cell background independently of the rest of the grid.
• Sticky positioning only activates inside a scrollable ancestor; the demo and docs show the required overflow-x: auto wrapper.

📑 Test Plan

13 new tests in FluentDataGridPinnedColumnTests.razor:

• Default Pin == None
• Single left/right pin: correct CSS class (col-pinned-left/col-pinned-right) and inline style (position: sticky; left/right: 0px)
• Two left-pinned columns: cumulative offsets (0px, 100px)
• Two right-pinned columns: cumulative offsets reversed (0px, 60px)
• Missing Width → ArgumentException
• Non-px Width (e.g. 1fr) → ArgumentException
• Left-pinned column not at start → ArgumentException
• Left-pinned columns non-contiguous → ArgumentException
• Right-pinned column not at end → ArgumentException
• Right-pinned columns non-contiguous → ArgumentException
• Valid mixed layout (left + normal + right) — assertion-based
• Snapshot test (verified HTML)

All 3099 existing tests continue to pass.

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

MCP Server

• I have added or updated MCP Server tools/prompts/resources
• I have added Unit Tests for my MCP Server changes

⏭ Next Steps

• Consider exposing UpdatePinnedColumnOffsets as a public JS interop method so host apps can trigger a re-sync after programmatic column visibility changes.
• RTL: left/right offsets are applied directly; verify visually in a full RTL demo once the component ships.