The Rust MT stream scheduler now copies input into the reusable round buffer
through its local projection and publishes the final filled count when the
outer call returns. Job creation, however, is a C callback invoked during that
same scheduler call. It still inspected `mtctx->inBuff.filled` while preparing
the job, so the count lagged behind the bytes Rust had just copied and the
existing preparation assertion fired in the multiple-file CLI path.
Publish the callback's `srcSize` into the C context before constructing the job
projection. This restores the ordering of the former C copy callback: the
context count is current before job preparation, while the existing end-of-call
result publication remains available for paths that do not create a job.
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
- `make -j1 -C tests test` reproduced the pre-fix assertion in
`ZSTDMT_prepareCompressionJob`; the capped full-suite rerun is pending
ddidderr
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