feat!: .NET 10, streaming writer, fixes, opt-in WriteExcelAsync API

[mmalkhatib]

Retargets the library to net10.0 + net8.0 and rewrites its internals as a streaming writer. Every existing method keeps its signature, column order and cell types. Merging publishes 10.0.0 to nuget.org.

Existing API: same output, but faster and fixed

WriteExcel / ExportToExcel / GenerateExcel now run on a writer that streams the xlsx parts straight into a zip, instead of building the whole sheet as an in-memory XML tree.

200k rows × 10 columns	8.0.6	this PR
Time	2.9 s	0.76 s
Allocated	1.4 GB	80 MB
Peak heap	945 MB	13 MB

Equivalence check: I compared 9 cases covering every overload and all supported types, about 15k cells, against 8.0.6, checking type, style and value for each cell. Numbers, dates, decimals and floats are the same text.

Differences from 8.0.6. Each was reproduced against 8.0.6. Where it says Excel refuses a file, the file was opened in Excel and failed.

• Control characters (\x01, \v …) in a string: used to throw XmlException mid-export. Now stripped.
• NaN/Infinity, text over 32,767 characters, or a type with no exportable properties: used to produce a file Excel refuses to open. Now written as text, truncated, and written as a valid empty sheet, respectively.
• Frozen header: the pane was written outside <sheetView>, so Excel ignored it. It now works.
• Column widths: every <col> element pointed at column 1, which made column A 4 characters wide. Widths are no longer forced.
• Empty strings: were written as a newline plus 10 spaces. Now an empty cell.
• An indexer property: threw TargetParameterCountException. Now skipped.
• Dictionary rows: values were matched by position, not by key, and an empty list threw. Now matched by key; an empty list gives an empty sheet.
• Column letters from column 677 on: were wrong ("A A"). Now correct.
• Dictionary sheets: now include a stylesheet.

New opt-in API: WriteExcelAsync

A separate method name, so existing callers are unaffected:

• Columns come out in the order given, from a property → header map or as computed ExcelColumn<T> lambdas.
• Rows can be IDictionary<string, object>, ExpandoObject or IDictionary<string, string>.
• IAsyncEnumerable<T> sources are supported (e.g. EF AsAsyncEnumerable()), with cancellation.
• Cells are typed: real booleans, dates, DateOnly, enums by name, Guid.
• Options: auto-fit widths, bold header, autofilter, date formats, sheet name.

Full details are in the README.

Compatibility notes

• DocumentFormat.OpenXml: the library no longer uses it, but it stays a dependency, now 3.5.1 instead of 2.13.1. Accounting, Checkout, Rates and Warehouse have stray using DocumentFormat.OpenXml… lines that compile only because this package brings it in. Dropping it broke all four builds.
• Traxis consumers: I built all five (Accounting, Checkout, Rates, Ship, Warehouse) from develop against this package. All build with no source changes. Ship resolves OpenXml 3.5.1 alongside ClosedXML 0.105.0, with no range warnings.
• Older frameworks: a project that can't move to .NET 8 can stay on 2.0.5 (netstandard2.0).

CI

• major-version goes 8 → 10.
• Installs the 8.0 and 10.0 SDKs.
• run-tests: true, running 34 tests on both frameworks. Each test reads the output back and runs it through OpenXmlValidator.
• MSTest 4.5.1 and Test.Sdk 18.10.1, which supersede Dependabot Bump Microsoft.NET.Test.Sdk from 17.14.1 to 18.10.1 #24, Bump MSTest.TestAdapter and MSTest.TestFramework #30 and Bump MSTest.TestFramework from 3.10.4 to 4.5.1 #31.

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