write_batch_to_disk reserves the full length when replacing an existing object #46

Follow-up from #45 review. write_batch_to_disk reserves len before the put while the previous copy still holds previous.bytes, and releases the old reservation only after the reserve succeeds. Replacing an object on a nearly full disk tier therefore needs both sizes free at once; the reserve fails, CacheFull propagates, and flush_all_to_disk drops an entry whose rewrite would not have grown the tier (flush_dropping_hydrated_entry_releases_its_disk_copy shows that outcome).

Fix: read the record before the reserve loop, reserve len - previous.bytes when the value grows and release previous.bytes - len after the put when it shrinks, keeping the record in place if the reserve fails so discard_disk_copy still matches a live reservation. The flush test then needs a different route to CacheFull.