The MT frame-progression callback is only referenced when ZSTD_MULTITHREAD is enabled. Keep its definition under the same configuration guard so single-threaded library, test, and decode-corpus builds do not report an unused static function while the MT callback remains available to the Rust dispatch bridge.
Test Plan:
- git diff --cached --check
- Capped full tests passed before this warning-only guard
- Capped native rebuild and smoke rerun pending
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