Move the post-call ZSTD_compressStream2_c policy into Rust: adapter errors now short-circuit before buffer publication, successful calls publish buffer expectations before calculating pending output, and the private C context remains behind a callback projection. Add ABI assertions and ordering tests for both paths.
Test Plan: git diff --cached --check; focused Rust tests added; full capped Rust/native/original-suite verification will run after the MT slice is integrated.
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