The multithreaded stream loop already delegated scalar result classification to
Rust, but its final error/end conditional still mixed completion ordering with
the private C context. That left trace and reset sequencing embedded beside the
MT counters and made the next Rust seam depend on exposing CCtx layout.
Rust now consumes the projected loop action and sequences the final completion
side effects through opaque C callbacks. Continue and break do nothing, worker
errors reset the session, and completed end operations trace before resetting.
The reset callback result is intentionally discarded. C retains the private
context mutations in callback adapters, and the callback invocation remains
inside the existing ZSTD_MULTITHREAD branch before FORWARD_IF_ERROR(flushMin).
C/Rust repr(C) projections and ABI assertions protect the callback/result
boundary, while focused tests cover all four loop actions and end ordering.
Test Plan:
- `git diff --cached --check` -- passed
- `rustfmt --edition 2021 --check rust/src/zstd_compress.rs` -- passed
- single-thread and `-DZSTD_MULTITHREAD` C syntax checks -- passed; existing warnings only
- `CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings` -- passed
- `CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --benches -- -D warnings` -- passed
- focused Rust test filter was attempted serially but is blocked by unrelated in-progress `rust/src/zstdmt_compress.rs` test ABI edits; no full test suites were run
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