Keep the C-owned compression job callback responsible for private buffer and job-state mutation, but return the range-active decision alongside its result. Rust now owns the outer scheduler policy that publishes or suppresses the reusable input count after job creation, including the error path. The focused seam test covers the callback-clears-range case without exposing the MT context layout.
Test Plan: Not run in this atomic commit; the capped serial Rust, native, smoke, and original test-suite verification follows.
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