test: add quote assertion helpers

[adamcavendish]

Summary

• Add assert_rendered! for exact CodeBlock render assertions through FileSpec.
• Add assert_quote! sugar for exact inline sigil_quote! snapshots.
• Cover configured languages, imports, width-sensitive rendering, and CodeFragment composition.
• Document exact quote testing in the sigil_quote! guide.

Tests

• cargo test --test assert_quote_tests -- --nocapture
• cargo test
• git diff --check

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