Reject entry reuse after cancelled unpack

[zaniebot]

Summary

• mark an entry unusable when an in-progress unpack future is dropped
• reject subsequent unpack and body-read attempts with an explicit error
• preserve ordinary completed success and error behavior

Why

Regular-file unpacking drains the entry's data queue into the unpack future. If that future is cancelled after consuming data, retrying the same entry can previously succeed while writing an empty or truncated file. The consumed archive data cannot be rolled back, so explicit poisoning prevents a plausible but incorrect retry.

Testing

• added a regression test that cancels after entry data is consumed and verifies that unpack and read retries fail explicitly
• cargo check --all --all-targets --all-features
• cargo test
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings