[Repo Assist] Add Deedle.ExcelWriter; rename Deedle.Excel.Reader → Deedle.ExcelReader

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Closes #714 — Add Deedle.ExcelWriter package
Closes #713 — Rename Deedle.Excel.Reader → Deedle.ExcelReader

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What's changed

New: Deedle.ExcelWriter (closes #714)

A new cross-platform Excel writing package backed by MiniExcel — one of the fastest and most memory-efficient .xlsx writers available on NuGet.

F# API:

open Deedle
open Deedle.ExcelWriter

// Write to first sheet
Frame.writeExcel "output.xlsx" df

// Write to a named sheet
Frame.writeExcelSheet "output.xlsx" "Results" df

// Write multiple sheets
Frame.writeExcelSheets "output.xlsx" (seq {
    yield ("Q1", q1Frame)
    yield ("Q2", q2Frame)
})

// Write to a stream
Frame.writeExcelStream myStream df
Frame.writeExcelSheetStream myStream "Sheet1" df

C# API:

ExcelFrameWriter.WriteExcel(path, frame);
ExcelFrameWriter.WriteExcelSheet(path, "Sheet1", frame);
ExcelFrameWriter.WriteExcelStream(stream, frame);

Missing values are written as null (empty cells). Any existing file is overwritten.

Fix: Rename Deedle.Excel.Reader → Deedle.ExcelReader (closes #713)

The NuGet package name Deedle.Excel.Reader was inconsistent with its namespace Deedle.ExcelReader. This PR renames the package, project, and test project to Deedle.ExcelReader. The namespace, API, and all F#/C# types are unchanged.

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Files changed

Change	Description
src/Deedle.ExcelReader/	Renamed from src/Deedle.Excel.Reader/
src/Deedle.ExcelWriter/	New package — ExcelWriter.fs + project file
tests/Deedle.ExcelReader.Tests/	Renamed from tests/Deedle.Excel.Reader.Tests/
tests/Deedle.ExcelWriter.Tests/	10 new tests for the writer
tests/Deedle.CSharp.Tests/	Updated project reference and file paths
docs/excel.fsx	Combined ExcelReader + ExcelWriter docs
docs/csharp.md, docs/index.fsx	Updated package name references
paket.dependencies	Added MiniExcel
RELEASE_NOTES.md	Added entries for both changes

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Test Status

All tests pass (dotnet test Deedle.sln -c Release):

• Deedle.ExcelReader.Tests: 20 passed
• Deedle.ExcelWriter.Tests: 10 passed
• Deedle.CSharp.Tests (Excel section): 8 passed
• Full suite: 844 tests, 0 failures

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