Keep C ownership of the block-state structure while delegating the confirmed-block pointer swap through a narrow two-pointer ABI leaf. Preserve every existing confirmation call site and add direct pointer-swap coverage.
Test Plan:
- cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression
- cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression --all-targets
- cargo +nightly fmt --manifest-path rust/Cargo.toml
- make -B -C lib -j2 lib
- tests/fuzzer -s4560 -t47 -i48 -v
- make -C tests -j2 test-zstream