Close libarchive archive/filter parity gaps

[Hawkynt]

What changed

Two libarchive formats CWB did not have:

• BSD mtree(5) manifest reader, writer and registry descriptor. mtree is treated as what it is — a metadata manifest with no embedded file bodies — so the descriptor advertises list/create/test and not extract, and CWB never dereferences a contents= host path out of an untrusted manifest.
• begin-base64 / ==== wrapper as B64Encoding, libarchive's b64encode filter: streaming 57-byte input blocks, canonical 76-character lines, and name/mode options.

The CPIO half of the original branch has been dropped. Read and write every historical cpio header variant (#376) landed all four header variants on main first, with a shared CpioLayout, verified crc checksums, variant-preserving edits and a larger test set. This branch no longer touches any CPIO file.

Fixes made while rebasing

• The wrapper announced a mode C# had already turned into a different number. 0644 and 0777 are decimal literals in C#, not octal, so the mask cleared permission bits and the default was never 0644: the header carried 1000 by default and 400 for a requested 0600. The mask and default are now named constants written in hex alongside the octal value they stand for.
• The same bug was live on main's classic uuencode path, which emitted begin 1204. libarchive rejects that header outright (Unrecognized archive format) and accepts the begin 644 now emitted, so this was an interoperability defect rather than a cosmetic one.
• The conversion matrix verified every conversion by reading the payload back, which assumes each creatable target stores bytes. mtree is the first target that does not, and the matrix read the absent payload as a broken conversion. Byte comparison is now skipped for a target that does not advertise CanExtract; the conversion still runs and the manifest is still re-listed and checked for the expected entry names. mtree is the only descriptor in the repository this applies to.

Not included

PWB/UNIX binary CPIO is deliberately left out. It is byte-identical to the little-endian 7th Edition variant, so the original branch detected it with a mode-bit heuristic that latched for the remainder of the archive and rewrote mode bits once it fired. Placing that in front of binary CPIO reads which #376 validated against bsdcpio and GNU cpio risks changing archives that read correctly today. If PWB is wanted, an explicit write option plus an explicit read override — the shape libarchive itself uses — is the safe design, and belongs in its own change.

Validation

• Gating suite: 0 failed, 30337 passed, 1446 skipped. Measured against main (0 failed, 30295 passed, 1446 skipped) test-for-test: no test present on main is missing or changed outcome; 42 are new.
• bsdtar / bsdcpio (libarchive 3.8.9) run as real oracles in both directions for the wrapper and for mtree.
• Build: 0 errors, 348 warnings, matching main.
• Package READMEs regenerated; the support-matrix rows for both new formats are checked against the live registry by ArchivesReadmeStateTests.

References

• FreeBSD/libarchive mtree(5)
• libarchive b64encode filter and its published test vector