fix: skip data reads on directory entries for RAR archives

[otavio]

Summary

• libarchive's RAR reader returns ARCHIVE_FAILED with "Can't decompress an entry marked as a directory" when archive_read_data_block is called on a directory entry (other formats just return ARCHIVE_EOF). Our extractors called it unconditionally, so any RAR with directory entries failed extraction — even after files had already been written to disk.
• Added a libarchive_entry_is_dir helper (reads stat.st_mode against POSIX S_IFMT/S_IFDIR so it stays Windows-safe, since libc::S_IFDIR isn't exposed there) and gate the data-read step on it in uncompress_archive_with_encoding, uncompress_archive_file_with_encoding (returns Ok(0) for dir paths), and ArchiveIterator::next_data_chunk (returns EndOfEntry).
• For the iterator, current_is_dir is cached on StartOfEntry so per-chunk reads don't re-dereference archive_entry_stat.

Closes #131.

Test plan

• New tests/fixtures/tree.rar (generated with rar -ma5, mirrors tree.tar) covering both file and directory entries.
• Confirmed the existing extraction path fails on tree.rar before the fix with the reported error.
• New integration tests pass after the fix: uncompress_rar_to_dir, get_a_file_from_rar, iterate_rar_entries.
• Full suite green: cargo test --features tokio_support,futures_support — 56 integration tests + 14 doc tests pass, no regressions.

[coveralls]

Coverage Report for CI Build 24834523391

Coverage increased (+0.5%) to 77.628%

Details

• Coverage increased (+0.5%) from the base build.
• Patch coverage: 2 uncovered changes across 2 files (57 of 59 lines covered, 96.61%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/lib.rs	10	9	90.0%
tests/integration_test.rs	44	43	97.73%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	2454
Covered Lines:	1905
Line Coverage:	77.63%
Coverage Strength:	84.73 hits per line

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💛 - Coveralls