45cab0620f
A sweep found no defect in the decoder and several in its tests. Three of the five fixes could be reverted with all 103 tests still green. The content size against expectedSize check is the worst of those, because the .hgtz reader was simplified to rest on it: deleting it left everything passing. Eleven cases cover it now. The RLE block ceiling and the requirement that a frame be the whole input had none either, so there are fixtures for a 13-byte frame that returns 2 MB, for the same shape at exactly the maximum, for four bytes of junk after a frame, and for a trailing skippable frame. Two of the new fixtures close coverage gaps rather than guards. No legal frame in the suite decoded a one-stream Huffman literals section, since the only one present had to be refused. And the only test of hashing each block as it lands rode on the zstd CLI being installed, so a multi-block frame carrying a checksum is committed now. Also replaces an assertion that constrained nothing. "Fills the buffer exactly" checked that the count was over 256 and under 1024, which passes with the guard at two, at three, or absent. The stream encodes 257 symbols, a buffer of 257 takes it and one of 256 does not. And two comments that were wrong. The weight buffer is one larger than the maximum on purpose, and shrinking it to 255 breaks 124 tests. A caller passing expectedSize gets the exact check in addition to the ceiling, not instead of it.
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