[MagicalTux]

Rebased onto master after #149 merged; this is now the single partial-chunk commit.

Problem

A compressed LZMA2 chunk (up to 64 KiB of input, 2 MiB of output) was only decoded once all of its bytes were buffered.

• Truncated streams lost the whole chunk the cut fell in. A 4 MB file cut 40 bytes short gave 2,096,914 bytes; xz -dc gives 3,968,329.
• Streaming callers got no output from a chunk until its last byte arrived.

Fix

Packets are now decoded as the compressed bytes arrive, the way liblzma does it.

• While the chunk is incomplete, a packet is decoded only when at least 21 unread bytes are buffered. That is liblzma's LZMA_IN_REQUIRED, the most input one packet can read: at most 22 probability-coded bits (each shrinks the range by at most about 6.05 bits) plus 26 direct bits, so 159 bits, read one byte per 8 bits of shrink, plus one. A packet therefore never runs out of input halfway and leaves the coder state half-updated.
• Once the chunk is complete, it decodes to the end.
• At finish on a stream cut inside a chunk, one last pass decodes every complete packet the bytes hold. A packet writes nothing until it has been read in full, so every byte before the cut is delivered, and then UnexpectedEnd is returned.

LzmaCore::decode_partial is new; decode_chunk now wraps it.

Verification

• Against xz -dc: truncated output matches byte for byte at 4 cut points in each of 4 streams: 2 MiB chunks of zeros, a binary, an x86 BCJ stream, and a SHA-256 multi-block stream.
• New tests, no xz tool needed: half of a compressed chunk already yields output when fed in 1-, 100- and whole-piece slices, and cuts inside a chunk recover more than the complete chunks hold, with any buffer size. When xz is installed, the second test also compares against xz -d. Both tests fail on master.
• Speed: full-stream decode is unchanged against master (0.037s vs 0.037s on a 4 MB binary).
• cargo test --features all: 1925 passed. clippy (-D warnings, all and --all-features) and fmt are clean.

Not changed

The raw lzma2 decoder (src/lzma2) still decodes whole chunks. It can use the same decode_partial, as a follow-up.

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