Reject builder reuse after cancelled append

[zaniebot]

Summary

• guard every public builder append operation against cancellation
• poison the builder when an append future is dropped in progress
• reject later appends and finalization instead of emitting a corrupt archive

Why

An append can write part of a header or payload before returning Pending. Dropping that future loses its write offset; retrying starts a new entry at the current sink position and can produce a corrupt archive while reporting success. Since a generic AsyncWrite sink cannot be rolled back, the builder must not be reused after cancellation.

Testing

• added a regression test that cancels after a partial header write and verifies that retry is rejected without further output
• cargo check --all --all-targets --all-features
• cargo test
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings