Document and test all scopes where #[mutants::skip] applies

[sandersaares]

Stacked on top of sourcefrog#607 (�xclude-re-attr).#[mutants::skip] has always been honoured at every scope where the visitor checks attributes — not just on fn declarations. The book's Skipping functions with an attribute section implied otherwise, and explicit test coverage of the attribute only exercised top-level functions, impl methods, trait default methods, and modules (via testdata trees), leaving every other supported scope unexercised.

This PR locks in the existing behaviour and clarifies the docs. No production code changes.

Tests added

New unit tests under src/visit/test/ for every previously-uncovered scope:

• #![mutants::skip] — file inner attribute
• #[mutants::skip] impl Foo { ... } — on the impl block itself
• #[mutants::skip] trait Foo { ... } — on the trait declaration itself
• expression-level skip on call, method-call, match, struct literal, and unary expressions
• #[cfg_attr(test, mutants::skip)] at non-fn scopes (impl, mod)

Skip on a binary expression is intentionally not tested: syn absorbs a leading attribute into the leftmost operand, so ExprBinary.attrs is unreachable from any current Rust syntax (same constraint that applies to mutants::exclude_re).

Tests follow the per-file layout already established by the exclude_re_expr_* modules.

Docs

book/src/attrs.md:

• Rename heading from Skipping functions with an attribute to Skipping mutations with an attribute and reword functions → items.
• Tighten the second list item to specify the only two recognised forms (direct #[mutants::skip] or nested inside cfg_attr); drop the misleading avoid the dependency sentence.
• Rewrite the inline Note to accurately describe the cfg-condition-not-evaluated behaviour rather than the previous looks for the sequence framing.
• Tighten the parallel paragraph in the exclude_re section the same way.
• Add a Scope subsection to the Skip section, mirroring the one for exclude_re.

[sandersaares]

Splitting this up: the exclude_re doc tightening goes into PR sourcefrog#607 (exclude-re-attr) directly; the mutants::skip docs + tests will go in a new independent PR based on main.