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7z: raw LZMA2 decoder entry point + BCJ2 filter #74

@MagicalTux

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@MagicalTux

Split from #65 (BCJ ×8 + Delta shipped in #68). Remaining 7z pieces:

1. Raw LZMA2 Decoder entry point (highest priority — 7z default, codec id 21).
7z's LZMA2 coder is a raw LZMA2 chunk stream (dict-reset control bytes + LZMA chunks), not the .xz container. The LZMA2 algorithm already lives in the xz module but isn't reachable for raw use. Expose a raw LZMA2 Decoder taking the 1-byte coder property (dict size) and the known unpack size, consumable via DecoderReader.

2. BCJ2 (0303011B, 4-stream).
The 4-input BCJ2 filter (distinct from the single-stream BCJ x86 03030103 already shipped).

Both decode-only, Decoder-shaped for a DecoderReader filter chain. Consumer: KarpelesLab/fstool 7z reader.

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