cargo-mutants over selection.rs found every branch mutation in
is_canonical surviving (the Q-J door was reachable only through the
handful of spellings validation_refuses_non_canonical_number_spellings
happened to cover), plus decompose/pad_right/canonicalize_decimal_digits/
big_mul_small/big_to_most_significant_first each replaceable with a
trivial stub while the suite stayed green.
Add tests that call is_canonical, decompose, pad_right, compare,
canonicalize_decimal_digits, big_mul_small and big_to_most_significant_first
directly (the nested test module already reaches private helpers via
super::), pinning exact output at the inputs that exercise each branch,
comparison and carry step named in the mutant list rather than only the
few spellings the field-validation door happened to already cover.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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