fix(builtins): sort -n extracts leading numeric prefix from strings

[chaliy]

Summary

• sort -n now parses the leading numeric portion of strings (e.g. 0003-msg.md → 3) instead of requiring the entire token to be a valid number
• Adds full-line fallback comparison when sort keys are equal, fixing field-based sorting (-t/-k) when keys tie
• Adds 7 new spec tests covering numeric prefix extraction, mixed prefix lengths, non-numeric-as-zero, field-based delimiter sorting, reverse numeric, and zero-padded numbers

Test plan

• New spec tests pass: sort_numeric_prefix_strings, sort_numeric_mixed_prefix_lengths, sort_numeric_nonnumeric_as_zero, sort_field_delim_k2, sort_field_delim_k1, sort_numeric_reverse, sort_numeric_zero_padded
• All existing sort/uniq spec tests still pass
• cargo fmt --check clean
• cargo clippy -p bashkit -- -D warnings clean

Closes #833