`write_full` can't know the output size in advance, so its buffer grows by
doubling and the result kept up to as much again unused: 28 % for a 100 MB
save, 99 % for a 5 MB one. The result is often kept for as long as a document
is open. It becomes the working bytes after edits that need a full rewrite
(repaired files, security changes, redaction) through `write_incremental`, and
it backs Save As ▸ optimized and every new document from Create, Combine,
Extract and Split.
`shrink_to_fit` at the end drops the unused capacity. The bytes are the same.
On macOS the shrink happens in place (2 µs for 100 MB, the buffer doesn't
move), and full-save times are unchanged.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
b98505c810