Add libarchive 3.8.1; make it a diffoscope runtime dep

[bryan-minimal]

What

Adds libarchive 3.8.1 (library-only) and makes it a runtime dep of diffoscope.

Why

diffoscope imports python's libarchive-c at startup — not lazily — which
dlopens the native libarchive.so. No package ships it, so every
diffoscope invocation dies at import, --version included:

AttributeError: /usr/bin/python3.14: undefined symbol: archive_version_number
Missing or incomplete libarchive module. Try installing your system's 'libarchive' package.

(The odd undefined symbol on the python binary itself is libarchive-c's
ctypes fallback after find_library('archive') returns nothing.)

Found on v0.5.0 release night by repro-lab's rc-tools, which invokes every
tool instead of trusting command -v — locating a binary is not evidence
that it runs.

Recipe notes

• One subtlety baked into the diffoscope wrapper: ctypes' find_library()
  has neither ldconfig nor gcc inside a composed root, so libarchive-c can't
  find the .so on its own even when present — it honors $LIBARCHIVE, so the
  console script ships behind a two-line wrapper that sets it (overridable).

• Library only: --disable-bsdtar/bsdcat/bsdcpio/bsdunzip — the tools aren't
  needed and every compressor skipped is a runtime dep not dragged in.

• Linked against zlib / bzip2 / zstd (already in pkgs); lz4, lzo2, lzma,
  xml2, expat, openssl, nettle, ACLs and xattrs configured out.

• No make check: the suite exercises the ACL/xattr/locale surface we
  deliberately configured out.

• Source staged at gs://minimal-staging-archives/libarchive-3.8.1.tar.gz,
  sha256 bde832a5e3344dc723cfe9cc37f8e54bde04565bfe6f136bc1bd31ab352e9fab
  (upstream GitHub release tarball).

Tested

On min 0.5.0, in a clean-room session on this branch:

• min package build libarchive — builds clean (autotools kit: sed/grep/gawk-bootstrap, same set as file)
• min package build diffoscope (spec hash changes with the new dep) — builds clean
• task running bare diffoscope --version in the composed root — diffoscope 325, first successful invocation of this package

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 02e5f82 and 96f3690.

📒 Files selected for processing (5)

• minimal.toml
• packages/diffoscope/build.ncl
• packages/diffoscope/build.sh
• packages/libarchive/build.ncl
• packages/libarchive/build.sh

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