Expanded archives keep their original dates

[danifunker]

Summary

When an archive is expanded into an image (--expand-archives, --expand-gunzip, on every optical new CD builder and rb-cli import), anything the archive does not date itself used to take the day of the import. Found while indexing a set of NeXTSTEP CDs: files had their 1990s dates, but 1,763 of 18,877 directories were dated the build day.

Now, for every archive expansion and every target filesystem:

• Undated directories take the archive file's date. That covers the folder an archive unpacks into (pkg.tar.gz -> pkg/) and implicit parents a tar only lists through a/b/file (Importer::implicit_dir_times; ensure_dir_at takes a date).
• Undated tar members (mtime 0) take the archive file's date too.
• Classic Mac archive members (.sit, .cpt, .hqx, ...) keep their own Mac dates. copy_into never carried dates; CopyOptions::preserve_times turns that on for archive expansion only, so rb-cli cp and Commander Mode still stamp the time of the copy.
• --expand-gunzip output takes the original date in the gzip header, else the .gz file's date.

Entries that carry their own date in the archive already kept it, and still do.

Also

• .gnutar.gz, .gtar.gz, .gnutar and .gtar are recognised when naming the unpack folder: MiscKit.2.0.5.s.gnutar.gz -> MiscKit.2.0.5.s/ (was the full file name).
• ImportStats::perms_applied counted any non-empty override, so a date alone was reported as a permission applied. It now counts mode and owner only (AttrOverrides::has_permissions).

Testing

• New tests: the unpack folder and implicit parents take the archive date while members keep theirs; an mtime-0 member takes the archive date; gunzip output uses the header date, and falls back to the .gz file's date; .gnutar / .gtar names strip.
• scripts/preflight.sh green: cargo test --release, MiSTer feature set, Rust 1.73 floor, doc parity, rb-regress.
• Rebuilt nine NeXTSTEP CD images with --expand-archives: all pass fsck with the same file / directory / symlink counts as before, directories dated the build day went from 1,763 to 0 apart from folders that come from the host tree itself, and files dated the build day went from 4 to 0.

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