The multithreaded branch of `ZSTD_compressStream2_c` performed the complete
post-call result classification in C: it prioritized errors, recognized a
completed end directive, and applied different progress rules for continue,
flush, and end operations. Those decisions were scalar policy around a C-owned
MT call, but remained embedded beside private counters, reset, and tracing.
Rust now classifies one projected iteration and returns an explicit action for
continue, break, error, or completed end. The C shim still owns the MT call,
input/output accounting, error forwarding, private context mutation, reset,
and trace callback; the existing condition ordering and progress comparisons
are preserved. ABI layout assertions and focused tests cover error precedence,
completed end, progress-based continue termination, and pending/full output.
Test Plan:
- `git diff --cached --check` -- passed
- `rustfmt --edition 2021 --check rust/src/zstd_compress.rs` -- passed
- Full capped Rust/native verification is the next serial step.
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