The MT streaming scheduler previously delegated each bounded input-buffer copy
through a C-only callback. That callback mutated the C context as a side
effect, which kept a trivial data movement operation outside the Rust stream
path and made the result projection incomplete.
Copy the checked input range directly in Rust with copy_nonoverlapping, after
the existing available-input, destination-capacity, and non-null-source checks.
The C input-range callback remains responsible for exposing the reusable buffer,
while the Rust result now returns its updated filled count. C publishes that
count back to mtctx after the scheduler call. The result ABI has explicit
cross-language layout assertions, and the scheduler test verifies both copied
bytes and fill accounting. Job scheduling and all other MT callbacks are
unchanged.
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
- `cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed
- `git diff --check` and `git diff --cached --check` -- passed
- Cargo build/test, make, fuzzers, and other heavy checks were not run per task instructions
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