[MagicalTux]

Decoder part of #156, with zlib's inflate as the reference.

API

• Error::NeedDictionary, a new variant (Error is #[non_exhaustive]).
• zlib::Decoder::dictionary_id() -> Option<u32>, needs_dictionary() -> bool, set_dictionary(&[u8]) -> Result<(), Error>. This is zlib's Z_NEED_DICT + inflateSetDictionary. A dictionary whose Adler-32 doesn't match the DICTID gives ChecksumMismatch and the decoder keeps waiting.
• deflate::Decoder, zlib::Decoder, gzip::Decoder:
  • decode_block(input, output): like inflate(Z_BLOCK), it stops after each non-final block, and after the header for zlib and gzip.
  • at_block_boundary(), in_final_block()
  • unused_bits(): like zlib's data_type & 7.

Behavior changes

• Exact consumed: when the stream ends, the bytes after it are never counted as consumed, for raw deflate, zlib and gzip (single- and multi-member). zlib and gzip read their trailers straight from the input; the carry-over buffer is gone.
• FDICT without a configured dictionary now returns NeedDictionary after reading the DICTID. Before, it returned Unsupported. The embed build still returns Unsupported.
• An empty stored block (as from a sync flush) now counts as progress, so the decoder no longer returns with the next block header unread.

Validation

• Static fixtures from CPython's zlib cover Z_BLOCK, sync, full and partial flushes, fixed Huffman, and a preset dictionary.
• Exact consumption is checked across trailing garbage and single/multi-member gzip, feeding input in 1, 2, 7 bytes or all at once, with 1/5/4096-byte outputs.
• Block stops are compared against an independent bit-level parser.
• A live cross-check runs 50 fresh Python zlib streams (levels 0–9, every strategy, every flush mode, window bits 9–15, dictionaries). It skips when python3 is missing.
• Decode speed is unchanged: 189–192 MB/s before and after, run interleaved.
• Rebased onto feat(deflate,zlib,gzip): level 0, strategies, mem level, window bits, preset dictionary, partial/block flush #164; both test suites pass together.

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