OPENSTEP support: open Sun-labelled NeXT CDs, expand .tar.bz2 on import

[danifunker]

Two changes from building the OPENSTEP CD-ROM set.

1. Open Sun-labelled NeXT CDs through their NeXT label

CDs that also boot on SPARC carry a Sun disklabel at block 0 (next to an HP-PA LIF header) and keep their NeXT dlV3 copies at 512-byte blocks 16 / 30 / 48 (bytes 8192 / 15360 / 24576). Partition detection checked for Sun first and returned on it, so rb-cli inspect showed "Partition table: Sun / No partitions detected" and the UFS could not be browsed or checked.

• The NeXT label probe now runs ahead of the Sun check. A NeXT copy is checksummed, so it cannot match on a real Sun disk; plain Sun disks still detect as Sun.
• The probe also looks at blocks 16 and 48. present_copies sees them too, so a label rewrite (restore resize, partition edits) updates every copy, not just the one at block 30.
• write_copies keeps the dl_label_blkno those two copies record (4 and 12, in 2048-byte units), matching the real disc.

Checked by hand with the release binary (inspect, fsck IMG@1) on every NeXT CD image available:

Disc	Layout	Result
NeXTSTEP 3.3 CISC (68k + Intel)	standard copies	NeXT, fsck clean (14,820 files)
NeXTTIME	standard copies	NeXT, fsck clean
NeXTSTEP 3.3 RISC	Sun + blocks 16/30/48	NeXT, fsck clean (13,629 files), was undetected before
OPENSTEP 4.2 Install	Sun + blocks 16/30/48	NeXT, fsck clean (20,019 files), was undetected before

New test a_sun_labelled_multi_arch_disc_opens_through_its_next_label in next_cd_builder.rs reshapes a built disc into that layout and checks detection, the copy stamps, and that the UFS opens, takes a file and passes fsck. No real ISO is committed.

2. Expand .tar.bz2 archives on import

--expand-archives copied .tar.bz2 files in verbatim because tar detection only recognised gzip and zstd. The five copies of that magic-byte check in src/fs/tar_import.rs are now one open_tar_stream helper, which also recognises bzip2, including multi-stream files as pbzip2 writes them. .tbz joins the suffixes stripped from unpack folder names. The bzip2 crate was already a dependency (0.6, and 0.5.2 in the vintage manifest), so nothing new is added. New test a_multi_stream_tar_bz2_expands_into_its_own_folder; CLI help and docs/cli-reference.md updated.

Used for the OPENSTEP set: Developer/egcs-1.1.2-2.s.tar.bz2 now unpacks to 5,838 entries with its original dates.

Checks

scripts/preflight.sh passed on each change (release tests, MiSTer feature set, Rust 1.73 vintage build, doc parity, rb-regress); clippy and the affected tests pass on the combined branch.

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