fix: prevent destructive overwrite of decompression directories

[fzlzjerry]

Closes #1043.

Summary

• remove the destructive overwrite choice when a decompression conflict target is a directory
• keep rename, merge, and skip for directory conflicts while preserving overwrite for actual file conflicts
• update the multi-format merge coverage and add regressions for preserving unrelated directory contents and overwriting a single-file target
• document the fix in the changelog

Validation

• cargo test --locked
• cargo test --locked --no-default-features
• cargo test --locked --no-default-features --features unrar,use_zlib,use_zstd_thin
• cargo clippy --locked --all-targets -- -D warnings
• cargo clippy --locked --no-default-features --all-targets -- -D warnings
• cargo +nightly fmt -- --check