Make ZSTDMT_createCompressionJob match the exact callback type consumed by
ZSTDMT_rust_compressStreamGeneric. The callback now performs the existing
opaque and end-directive casts itself, so the redundant C forwarding shim can
be removed without changing scheduling, assertions, logging, or ABI behavior.
Test Plan:
- `cc -fsyntax-only -Werror=incompatible-pointer-types -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -Ilib/legacy lib/compress/zstdmt_compress.c` -- passed.
- `git diff --check` and `git diff --cached --check` -- passed.
- No cargo, make, native, fuzz, or other heavy tests run per task scope.
ddidderr
f3dad84d86