Add field compared/ignored checking to Check.Equality

[AnthonyLloyd]

Extend Check.Equality with an optional fluent fields builder to verify a type's equality contract at the field level:

• Compared fields must change equality; ignored fields must not.
• Completeness: any undeclared field that affects equality is detected.
• New IEqualityComparer overload to test a comparer directly.
• Setters can be functional (record with) or in-place Action; a comparer and/or generator handle fields with normalized equality.
• Failures name the field from the setter expression.

Add README "Equality testing" section and tests covering compared, ignored, missing, mutable, nested, comparer and normalized cases.

[Copilot]

Pull request overview

This PR extends CsCheck’s Check.Equality support to optionally validate equality at a field level (compared vs ignored fields, plus completeness detection for undeclared fields), adds an IEqualityComparer<T> overload, and documents the new feature with accompanying tests.

Changes:

• Add optional “fields contract” checking to Check.Equality, including compared/ignored field validation and missing-field detection.
• Add an overload to validate an IEqualityComparer<T> directly using the same consistency and optional field-contract checks.
• Add documentation and tests covering compared/ignored/missing fields, mutable setters, nested fields, custom comparers, and normalized equality.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
CsCheck/Check.cs	Adds field-level equality contract checking, comparer overload, and related helper generators/types.
Tests/CheckTests.cs	Adds tests that exercise the new field contract functionality and comparer/normalized cases.
README.md	Documents the new “Equality testing” feature and provides examples.

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[AnthonyLloyd]

@copilot do you think the readme description is good. I like what you put in this PR summary as it was simple. Is the REAME section too wordy?

[Copilot]

@copilot do you think the readme description is good. I like what you put in this PR summary as it was simple. Is the REAME section too wordy?

I agree it was a bit wordy, so I tightened that section to match the simpler style from the PR summary while keeping the same technical content. Updated in 876cdbb.

[AnthonyLloyd]

@copilot can you move Equality_Int in the readme to before the Account type

[Copilot]

@copilot can you move Equality_Int in the readme to before the Account type

Done in cdb6c3d — Equality_Int is now before the Account type in the README example.

[AnthonyLloyd]

@copilot " if an undeclared field affects equality, it fails and names the field from the setter expression." how does it get a setter if undeclared?

[Copilot]

@copilot " if an undeclared field affects equality, it fails and names the field from the setter expression." how does it get a setter if undeclared?

You’re right — it can’t for undeclared fields. I’ve updated the README to say undeclared-field failures are generic, and setter-expression naming applies to declared fields. Updated in fe9a2b3.