Complete the reproducible CI toolchain

[dv00d00]

Summary

• pin the .NET 10 feature band, CSharpier 1.3.0, GitHub Actions, and the compatible AngleSharp source commit
• verify in CI that AngleSharp resolves through the paired source project rather than the package
• remove the hard-coded sibling AngleSharp project from the solution so AngleSharpSourceRoot is authoritative
• add independent formatting, full net10 build/test/report, weekly, and .NET 11 runtime-async sample gates
• normalize the existing formatter drift in a separate mechanical commit

Validation

• local tool restore resolves CSharpier 1.3.0; 236 / 236 files pass formatting
• pinned AngleSharp revision 4819b43afb663ba29d37eb4f09abd072fed1966e is the only AngleSharp source built
• complete Release solution build: 0 warnings, 0 errors
• net10.0 tests: 179,562 / 179,562 passed
• TRX report generation verified

The .NET 11 preview lane is intentionally validated by GitHub Actions because the preview SDK is not installed locally.

Closes #57