Add gradient fill support to resolve EPPlus color reading limitations

[Copilot]

This PR adds comprehensive gradient fill support to ImportExcel, addressing the EPPlus 4.x limitation where gradient colors can be set but cannot be read back, preventing gradient copying workflows.

Problem

Users reported (issue #1720) that while gradient colors could be set using EPPlus:

$Sheet.Cells["A1"].Style.Fill.Gradient.Color1.SetColor("BLUE")
$Sheet.Cells["A1"].Style.Fill.Gradient.Color2.SetColor("GREEN")

Reading back the values failed:

$Sheet.Cells["A1"].Style.Fill.Gradient.Color1.Rgb  # Returns empty string

This made it impossible to copy gradients between cells, as the color information was lost.

Solution

Added three new functions that work around EPPlus limitations:

Set-ExcelGradientFill

Clean interface for applying gradient fills with full parameter validation:

Set-ExcelGradientFill -Range $ws.Cells["A1"] -Type Linear -Degree 90 -Color1 Blue -Color2 Green

Get-ExcelGradientFill

Reads gradient properties that can be reliably extracted from EPPlus:

$gradient = Get-ExcelGradientFill -Range $ws.Cells["A1"]
# Returns: Type, Degree, positioning (colors are null with clear explanation)

Copy-ExcelGradientFill

Enables gradient copying via two approaches:

1. Copy geometric properties only (with warning about colors)
2. Apply complete gradient definitions including colors

# Complete gradient copying
$gradientDef = @{
    Type = "Linear"
    Degree = 45
    Color1 = [System.Drawing.Color]::Blue
    Color2 = [System.Drawing.Color]::Green
}
Copy-ExcelGradientFill -TargetRange $ws.Cells["B1:D3"] -GradientDefinition $gradientDef

Key Features

• Comprehensive parameter validation with clear error messages
• Supports both string color names and Color objects
• Proper warning messages about EPPlus limitations
• Full test coverage (13 passing tests)
• Detailed documentation including migration guide
• Backwards compatible - no changes to existing functionality

Files Added/Modified

• Public/Set-ExcelGradientFill.ps1 - New gradient setting function
• Public/Get-ExcelGradientFill.ps1 - New gradient reading function
• Public/Copy-ExcelGradientFill.ps1 - New gradient copying function
• __tests__/GradientFill.tests.ps1 - Comprehensive test suite
• GRADIENT_GUIDE.md - Detailed usage documentation
• ImportExcel.psd1 - Updated to export new functions
• README.md - Added gradient feature documentation

The solution transforms an impossible workflow into a straightforward process while maintaining clear communication about underlying EPPlus limitations.

Fixes #1720.

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