The read handler now composes full directory paths from the cdir's
directory ids rather than emitting bare leaf names: build_dir_path
walks the parent chain (root dirid 0, depth-capped against cyclic
cdirs), so multi-file archives with subdirectories list correctly.
Master-block resolution (M4) and tagged long names (M6) already work
through libuc2's extract and tag paths; this adds a libarchive
round-trip test that creates archives at Huffman and rANS levels and
verifies every byte back through libarchive's public API. Documents
the plugin build recipe (libarchive source tree + static lib).
Verified against libarchive 3.7.7; round-trip clean under valgrind.
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