Move the public ZSTD_getFrameProgression MT-versus-single-thread worker-count
branch into a Rust-owned scalar projection. C continues to compute the
single-thread scalar inputs and keeps the private ZSTDMT context behind a
callback; the compile-time ZSTD_MULTITHREAD adapter remains local to C. Add
C/Rust layout assertions and focused Rust tests for both dispatch paths and
null-state fallback.
Test Plan:
- `git diff --cached --check`
- Not run: Cargo, make, builds, native tests, and heavy verification per request.
ddidderr
11dc99899b