Build a zstd corpus for the branches real tiles miss

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.

test/fixtures/corpus/manifest.json

@@ -0,0 +1,157 @@
+[
+  {
+    "name": "twoframes",
+    "frame": "twoframes.zst",
+    "command": "python3: pyzstd.compress(A, 3) + pyzstd.compress(B, 3), A = 5000 xorshift64 bytes, B = b'the quick brown fox ' * 500",
+    "verdict": "legal",
+    "plain": [
+      "A.raw.gz",
+      "B.raw.gz"
+    ],
+    "size": 15000,
+    "trailing": "a second complete frame"
+  },
+  {
+    "name": "checksum_good",
+    "frame": "checksum_good.zst",
+    "command": "zstd -q -c -3 --check A.bin",
+    "verdict": "legal",
+    "plain": [
+      "A.raw.gz"
+    ],
+    "size": 5000
+  },
+  {
+    "name": "checksum_flipped",
+    "frame": "checksum_good.zst",
+    "command": "zstd -q -c -3 --check A.bin, then the last checksum byte flipped",
+    "verdict": "refused",
+    "why": "pyzstd refuses it: ZstdError",
+    "derive": "the last checksum byte flipped"
+  },
+  {
+    "name": "checksum_zeroed",
+    "frame": "checksum_good.zst",
+    "command": "zstd -q -c -3 --check A.bin, then all four checksum bytes zeroed",
+    "verdict": "refused",
+    "why": "pyzstd refuses it: ZstdError",
+    "derive": "all four checksum bytes zeroed"
+  },
+  {
+    "name": "checksum_truncated",
+    "frame": "checksum_good.zst",
+    "command": "zstd -q -c -3 --check A.bin, then the last two bytes cut",
+    "verdict": "refused",
+    "why": "pyzstd refuses it: ZstdError",
+    "derive": "the last two bytes cut"
+  },
+  {
+    "name": "fcs_max_u64",
+    "frame": "fcs_max_u64.zst",
+    "command": "python3: pyzstd.compress(b'hello world', 3) with the descriptor rewritten to Frame_Content_Size_flag=3 declaring 18446744073709551615",
+    "verdict": "refused",
+    "why": "a 27-byte frame asking for a size no allocator will give; pyzstd refuses it: ZstdError"
+  },
+  {
+    "name": "fcs_2p40",
+    "frame": "fcs_2p40.zst",
+    "command": "python3: pyzstd.compress(b'hello world', 3) with the descriptor rewritten to Frame_Content_Size_flag=3 declaring 1099511627776",
+    "verdict": "refused",
+    "why": "a 27-byte frame asking for a size no allocator will give; pyzstd refuses it: MemoryError"
+  },
+  {
+    "name": "fcs8",
+    "frame": "fcs8.zst",
+    "command": "python3: pyzstd.compress(B, 3) with the descriptor rewritten to Frame_Content_Size_flag=3 and an 8-byte size",
+    "verdict": "legal",
+    "plain": [
+      "B.raw.gz"
+    ],
+    "size": 10000
+  },
+  {
+    "name": "huf_weight_overrun",
+    "frame": "huf_weight_overrun.zst",
+    "command": "hand-built: a compressed block whose Huffman weight description is [0x10, 0x3F] and whose FSE weight stream is 33 x 0xAA then 0x02",
+    "verdict": "refused",
+    "why": "the weight stream decodes past the 256th weight; pyzstd refuses it: ZstdError"
+  },
+  {
+    "name": "rlelit_fmt0_R31",
+    "frame": "rlelit_fmt0_R31.zst",
+    "command": "hand-built: one compressed block, RLE literals Size_Format=0, Regenerated_Size=31, byte 0x5A, Number_of_Sequences=0",
+    "verdict": "legal",
+    "plain": [
+      "rlelit_fmt0_R31.raw.gz"
+    ],
+    "size": 31
+  },
+  {
+    "name": "rlelit_fmt1_R4000",
+    "frame": "rlelit_fmt1_R4000.zst",
+    "command": "hand-built: one compressed block, RLE literals Size_Format=1, Regenerated_Size=4000, byte 0x5A, Number_of_Sequences=0",
+    "verdict": "legal",
+    "plain": [
+      "rlelit_fmt1_R4000.raw.gz"
+    ],
+    "size": 4000
+  },
+  {
+    "name": "rlelit_fmt2_R70000",
+    "frame": "rlelit_fmt2_R70000.zst",
+    "command": "hand-built: one compressed block, RLE literals Size_Format=2, Regenerated_Size=70000, byte 0x5A, Number_of_Sequences=0",
+    "verdict": "legal",
+    "plain": [
+      "rlelit_fmt2_R70000.raw.gz"
+    ],
+    "size": 70000
+  },
+  {
+    "name": "rlelit_fmt2_R131072",
+    "frame": "rlelit_fmt2_R131072.zst",
+    "command": "hand-built: one compressed block, RLE literals Size_Format=2, Regenerated_Size=131072, byte 0x5A, Number_of_Sequences=0",
+    "verdict": "legal",
+    "plain": [
+      "rlelit_fmt2_R131072.raw.gz"
+    ],
+    "size": 131072
+  },
+  {
+    "name": "mixed_rle_raw",
+    "frame": "mixed_rle_raw.zst",
+    "command": "hand-built: [RLE 5000 x 0xAB][raw 33 bytes][RLE 40000 x 0x07] in one frame",
+    "verdict": "legal",
+    "plain": [
+      "mixed_rle_raw.raw.gz"
+    ],
+    "size": 45033
+  },
+  {
+    "name": "nofcs_windowed",
+    "frame": "nofcs_windowed.zst",
+    "command": "zstd -3 --no-check from stdin over b'the quick brown fox jumps over the lazy dog. ' * 24000",
+    "verdict": "legal",
+    "plain": [
+      "text.raw.gz"
+    ],
+    "size": 1080000,
+    "expectedSize": 1080000
+  },
+  {
+    "name": "direct_nibble",
+    "frame": "direct_nibble.zst",
+    "command": "block 183 of path/to/srtm1z/N00E042.hgtz, lifted out as its own frame; its literals carry the direct-nibble Huffman weight form",
+    "verdict": "legal",
+    "plain": [
+      "direct_nibble.raw.gz"
+    ],
+    "size": 131072
+  },
+  {
+    "name": "dictflag",
+    "frame": "dictflag.zst",
+    "command": "python3: pyzstd.compress(A, 3) with Dictionary_ID_flag forced to 1 in the frame header descriptor",
+    "verdict": "refused",
+    "why": "a dictionary this decoder does not have; pyzstd refuses it: ZstdError"
+  }
+]