fix: silence pulsar warnings under no-compression feature sets

[merlimat]

Problem

Building the pulsar crate without the compression feature — e.g.

cargo clippy -p pulsar --no-default-features --features tokio-runtime -- -D warnings

produces three warnings that are masked under the default feature set (which enables compression):

• src/producer.rs — unused io::Write import
• src/producer.rs — unnecessary mut on compress_message's message parameter
• src/consumer/engine.rs — unnecessary mut on process_payload's payload parameter

CI stays green today because both clippy jobs enable compression, but the warnings surface for any downstream build that depends on pulsar with default-features = false and no compression features.

Fix

Gate the import and the two mut bindings on the individual compression sub-features (lz4/flate2/zstd/snap) that actually use them — rather than the umbrella compression feature — so the crate is warning-free across every feature combination, including single-sub-feature builds like --features lz4, without changing behavior under any feature.

io::Write is moved out of the module-level use block into the flate2/snap match arms (the only two that call write_all), matching the existing use std::io::Read; convention already used for decompression in engine.rs.

No behavior change: under a no-compression build the only reachable match arm moves the message/payload without mutating it, so the mut is genuinely unused there.

Verification

• cargo clippy -p pulsar --no-default-features --features tokio-runtime -- -D warnings — now clean (previously 3 warnings)
• cargo clippy -p pulsar --features telemetry -- -D warnings — clean (compression-on path)
• cargo fmt --all --check — clean