petl-developers / petl / 42974102150

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Preserve existing data when appending to MemorySource (#796)

## Changes

Appending CSV rows to a preloaded `MemorySource` can discard the
original rows or overwrite them after a partial read. Keep the initial
data and move the write position to the end before each append.

## How it was tested

- All three new cases fail on unchanged `master` and pass with the fix.
They cover repeated CSV appends with and without a partial read, plus
text appends.
- `PYTHONHASHSEED=42 python -m pytest -q --cov=petl petl` on Python 3.12
and Linux: 1,310 passed, 25 skipped, and two database cleanup warnings.
Python 2 and service-backed test environments were outside this run.
- `git diff --check` passed.

## Checklist

- [x] Includes unit tests
- [x] Updated the docstring and `docs/changes.rst`
- [x] Single commit based on current `master`
- [x] Tested locally using `pytest`
- [ ] Upstream CI and coverage checks
- [x] Ready to review

20 of 20 new or added lines in 2 files covered. (100.0%)

16654 of 17778 relevant lines covered (93.68%)

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