test(mutation): Introduce mutation-based test coverage

[Eugene (talagrand)]

Test coverage improvements via cargo mutants

[Copilot]

Pull request overview

Adds mutation-testing-driven test coverage improvements (via cargo-mutants) and wires up CI support to report mutation outcomes on PRs.

Changes:

• Expand interpreter/parser/unit tests to cover additional error paths and equality/roundtrip behaviors (Scheme, JSONLogic, evaluator, builtins, AST).
• Add mutants.out/ to .gitignore and introduce a GitHub Actions workflow to run cargo mutants on PR diffs and comment results.
• Tighten a few existing tests to assert more specific error messages.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/scheme.rs	Adds parser tests for nested arity errors and longhand (quote ...) depth-limit coverage.
src/jsonlogic.rs	Adds tests for invalid var names and more AST→JSONLogic roundtrip cases.
src/evaluator.rs	Makes several error-case assertions more specific (lambda/define validation).
src/builtinops.rs	Extends equality coverage for distinct builtins.
src/ast.rs	Adds additional PartialEq coverage for function/env differences and PrecompiledOp args differences.
.gitignore	Ignores mutation testing output directory mutants.out/.
.github/workflows/mutants.yml	New workflow to run cargo-mutants on PR diffs and post a summary comment/artifact.

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[Copilot]

The workflow uses the Issues API (listComments/createComment/updateComment), but the job-level permissions only grants contents: read and pull-requests: write. With these permissions, GITHUB_TOKEN won’t have issues: write, so the PR comment step is likely to fail with a permissions error. Add issues: write (and optionally drop pull-requests: write if it’s not needed).

Suggested change

	pull-requests: write
	pull-requests: write
	issues: write