RYW transactions don't handle append_if_fits correctly

```
$ cat test.py && echo "===" && python3 test.py
import fdb

fdb.api_version(730)
db = fdb.open()

class Cancel(Exception):
pass

@fdb.transactional
def do_setup(tr):
tr[b""] = b"a" * 70000

do_setup(db)

@fdb.transactional
def do_test(tr, write_first):
if write_first:
do_setup(tr)
tr.append_if_fits(b"", b"b" * 20000)
tr.append_if_fits(b"", b"b" * 20000)
print(len(tr[b""]))
raise Cancel()

try:
do_test(db, False)
except Cancel:
pass

try:
do_test(db, True)
except Cancel:
pass
===
70000
90000
```

Start by running the reproduction in test.py and compare the two do_test paths, with and without write_first. Trace the transaction handling behind append_if_fits for read-your-writes behavior; done means both cases handle the append consistently and have regression coverage.

Written by the indexing model from the issue text.