Simplify wheel entry-writer ownership

[zaniebot]

The private EntryWriter is always constructed with a live EntrySeekableWriter, and close consumes the wrapper. Store the inner writer directly instead of modeling an unreachable closed state with Option. write and flush delegate to the same live writer, while explicit close still completes the entry before its RECORD row is added.

[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks

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_{Comparing zb/simplify-wheel-entry-writer (c6ce9d0) with main (c0df400)}