Switch zstd encoder from simple API (ZSTD_compress) to advanced API
(ZSTD_CCtx + ZSTD_compress2) with configurable worker threads via
ZSTD_c_nbWorkers. Consumer controls threading through threads=N
option string parameter. Default is 1 (single-threaded, bit-identical
output to previous behavior).
- Enable ZSTD_MULTITHREAD compile definition and link pthreads
- Add num_threads field to options struct and context
- Parse threads=N in option string (clamped >= 1)
- Rewrite aaruf_zstd_encode_buffer() with num_threads parameter
- Update all call sites (write.c, close.c)
Implements zstd as an alternative to LZMA for data block and
subchannel compression using the allocated compression IDs 4
(kCompressionZstd) and 5 (kCompressionZstdCst).
Adds zstd v1.5.7 as a bundled submodule with a static library
build including x86_64 assembly fast path for decompression.
Write path selects zstd or LZMA based on the zstd option.
Subchannel blocks use zstd+CST instead of LZMA+CST when enabled.
LZMA properties header is only written for LZMA-based compression
types, preventing format corruption in zstd-compressed images.
Activated via options string: "zstd=true;zstd_level=19".
Default remains LZMA for backwards compatibility.
Tested: SHA-256 verified lossless roundtrips across 9 disc systems
(Dreamcast, Saturn, Mega CD, PC Engine CD, Neo Geo CD, PS1, PS2 CD,
PS2 DVD) and PS1 SBI subchannel preservation with zstd+CST.
