lzma_compressor.cpp targets the 7-Zip LZMA SDK (LzmaEnc.h/Alloc.h), but
the build only finds xz/liblzma, which does not ship those headers, so
every build (release logs: 'Has header "LzmaEnc.h" ... NO') compiled it
out. No build could read or write LZMA blocks, and its decoder never
bounded output by the expected size.
The policy still chose LZMA for cold/archival and large inactive data;
compression then failed ('Compressor not available') and batch stores
kept those blocks uncompressed. Select Zstandard, which already uses the
archive level for old data.
Remove the compressor, its test, the enable-lzma option and the build
flags that passed it. The LZMA algorithm id stays in the header format,
and compression.lzma_level remains accepted in config (now unused).
