Fix doc gaps, add a package description, extend license year

[tgharold]

Motivation

The docs fell behind the 3.0 walk changes. The README said tuples are walked in one section and not walked in another. Its opening paragraph linked the ASP.NET Core 3.1 docs, and its usage sample passed an undefined variable. The package has never had a description, so nuget.org shows "Package Description". The LICENSE year for Thomas Harold covers only 2020, but work shipped in 2020, 2022 and 2026.

Summary

• LICENSE: Thomas Harold's notice is now 2020-2026. Mike Reust's line is unchanged.
• README: the Tuple classes are walked and value tuples are not. Each section now names the type it means. The opening paragraph says what Validator.TryValidateObject leaves out, and the usage sample is a C# code block.
• Package: add a <Description>.
• CHANGELOG: entries for the description and the README. Unreleased now lists Fixed after Changed, the order CLAUDE.md gives. A bullet that described no change is folded into the fix it explains.
• CLAUDE.md: describe the breadth-first walk, the 128-level limit and TypeExtensions.cs in place of the old recursive description.

No library code changes.

Test plan

• dotnet pack puts the new description in the nuspec
• Every README link resolves (GitHub blob links checked against the tree, because GitHub throttles curl on blob pages)
• Tuple statements checked against IsWalkedType and the existing Tuple_of_objects_is_still_validated test
• git diff --check is clean
• CI build and tests pass