fix: use last occurrence in combined short flags

[cuishuang]

Summary

Use the last occurrence of -p, -n, and -b when determining the effective option in combined short arguments.

The raw argument scanner currently uses Iterator::position, which finds the first occurrence of each flag. This works for combinations such as -pn and -np, but produces incorrect results when a flag occurs more than once.

For example, -npn ends with -n, so numbering should be enabled. The scanner currently compares the first n with p and incorrectly allows the loop-through printer to be selected, which drops the requested line numbers.

The same issue affects -b combinations:

• -pbp should use plain output.
• -bpb should number non-blank lines.

-pnp already produces plain output through clap's option handling, but the raw scan still selects the wrong internal printing path.

Switch the scans to rposition() and add regression coverage for all four combinations. The existing rule that -b takes precedence over -n remains unchanged.

Tests

• cargo test --test integration_tests
• cargo fmt --all -- --check