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Build a zstd corpus for the branches real tiles miss

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The only fixture was one .hgtz container written by one producer at level 19.
An audit ran 16,490 real blocks through an instrumented decoder and found
thirteen branches of RFC 8878 that corpus never reaches at all: raw blocks,
RLE blocks, RLE literals, treeless literals at the three-byte size format,
frames with no content size, frames with a checksum, the one and eight byte
content sizes, the three-byte sequence count, a block with no sequences, and
RLE sequence tables for literal and match length.

make_corpus.py builds frames for them, either through libzstd or by hand and
then past pyzstd for a verdict, so legal and malformed are libzstd's opinion
rather than ours. The expected plain bytes of a legal frame are whatever
pyzstd decompressed it to, gzipped to keep the fixture small, and the
manifest records the exact command behind every frame. 46 KB in all.

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